chrome: Support top offset for dialogs/popups with frameless window

[magreenblatt]

Is your feature request related to a problem? Please describe.
By default, Chrome/Chromium shows dialogs/popups immediately underneath the address bar. Frameless windows often do not have an address bar or title bar so dialogs/popups are shown aligned with the top of the window. It would be better to offset these windows either explicitly or implicitly, similar to #3461.

Describe the solution you'd like
Implement implicit offset logic similar to #3461.

Describe alternatives you've considered
Add a new CefWindowDelegate callback for retrieving the top window offset.

[magreenblatt]

Dialog position is retrieved via BrowserViewLayout::WebContentsModalDialogHostViews::GetDialogPosition.

For alert box (load https://tests/dialogs, click "Show Alert" button)

>	libcef.dll!BrowserViewLayout::WebContentsModalDialogHostViews::GetDialogPosition(const gfx::Size & size) Line 106	C++
 	libcef.dll!constrained_window::`anonymous namespace'::UpdateModalDialogPosition(views::Widget * widget, web_modal::ModalDialogHost * dialog_host, const gfx::Size & size) Line 132	C++
 	libcef.dll!constrained_window::UpdateWebContentsModalDialogPosition(views::Widget * widget, web_modal::WebContentsModalDialogHost * dialog_host) Line 185	C++
 	libcef.dll!constrained_window::NativeWebContentsModalDialogManagerViews::OnPositionRequiresUpdate() Line 158	C++
 	libcef.dll!constrained_window::NativeWebContentsModalDialogManagerViews::HostChanged(web_modal::WebContentsModalDialogHost * new_host) Line 209	C++
 	components_web_modal.dll!web_modal::WebContentsModalDialogManager::ShowDialogWithManager(aura::Window * dialog, std::__Cr::unique_ptr<web_modal::SingleWebContentsDialogManager,std::__Cr::default_delete<web_modal::SingleWebContentsDialogManager>> manager) Line 43	C++
 	libcef.dll!constrained_window::ShowModalDialog(aura::Window * dialog, content::WebContents * web_contents) Line 26	C++
 	libcef.dll!constrained_window::ShowWebModalDialogViews(views::WidgetDelegate * dialog, content::WebContents * initiator_web_contents) Line 216	C++
 	libcef.dll!JavaScriptTabModalDialogViewViews::JavaScriptTabModalDialogViewViews(content::WebContents * parent_web_contents, content::WebContents * alerting_web_contents, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> & title, content::JavaScriptDialogType dialog_type, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> & message_text, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> & default_prompt_text, base::OnceCallback<void (bool, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> &)> dialog_callback, base::OnceCallback<void ()> dialog_force_closed_callback) Line 124	C++
 	libcef.dll!JavaScriptTabModalDialogManagerDelegateDesktop::CreateNewDialog(content::WebContents * alerting_web_contents, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> & title, content::JavaScriptDialogType dialog_type, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> & message_text, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> & default_prompt_text, base::OnceCallback<void (bool, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> &)> dialog_callback, base::OnceCallback<void ()> dialog_force_closed_callback) Line 155	C++
 	libcef.dll!javascript_dialogs::TabModalDialogManager::RunJavaScriptDialog(content::WebContents * alerting_web_contents, content::RenderFrameHost * render_frame_host, content::JavaScriptDialogType dialog_type, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> & message_text, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> & default_prompt_text, base::OnceCallback<void (bool, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> &)> callback, bool * did_suppress_message) Line 161	C++
 	content.dll!content::WebContentsImpl::RunJavaScriptDialog(content::RenderFrameHostImpl * render_frame_host, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> & message, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> & default_prompt, content::JavaScriptDialogType dialog_type, bool disable_third_party_subframe_suppresion, base::OnceCallback<void (bool, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> &)> response_callback) Line 7663	C++
 	content.dll!content::RenderFrameHostImpl::RunJavaScriptDialog(const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> & message, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> & default_prompt, content::JavaScriptDialogType dialog_type, bool disable_third_party_subframe_suppresion, base::OnceCallback<void (bool, const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> &)> ipc_response_callback) Line 5634	C++
 	content.dll!content::RenderFrameHostImpl::RunModalAlertDialog(const std::__Cr::basic_string<char16_t,std::__Cr::char_traits<char16_t>,std::__Cr::allocator<char16_t>> & alert_message, bool disable_third_party_subframe_suppresion, base::OnceCallback<void ()> response_callback) Line 5579	C++

For modal dialog (select Tests > Dialog Window)

>	libcef.dll!BrowserViewLayout::WebContentsModalDialogHostViews::GetDialogPosition(const gfx::Size & size) Line 106	C++
	libcef.dll!constrained_window::`anonymous namespace'::UpdateModalDialogPosition(views::Widget * widget, web_modal::ModalDialogHost * dialog_host, const gfx::Size & size) Line 129	C++
 	libcef.dll!constrained_window::UpdateWebContentsModalDialogPosition(views::Widget * widget, web_modal::WebContentsModalDialogHost * dialog_host) Line 185	C++
 	libcef.dll!constrained_window::NativeWebContentsModalDialogManagerViews::OnPositionRequiresUpdate() Line 158	C++
 	libcef.dll!constrained_window::NativeWebContentsModalDialogManagerViews::HostChanged(web_modal::WebContentsModalDialogHost * new_host) Line 209	C++
 	components_web_modal.dll!web_modal::WebContentsModalDialogManager::ShowDialogWithManager(aura::Window * dialog, std::__Cr::unique_ptr<web_modal::SingleWebContentsDialogManager,std::__Cr::default_delete<web_modal::SingleWebContentsDialogManager>> manager) Line 43	C++
 	libcef.dll!constrained_window::ShowModalDialog(aura::Window * dialog, content::WebContents * web_contents) Line 26	C++
 	libcef.dll!CefWindowImpl::ShowAsBrowserModalDialog(scoped_refptr<CefBrowserView> browser_view) Line 147	C++
 	libcef.dll!`anonymous namespace'::window_show_as_browser_modal_dialog(_cef_window_t * self, _cef_browser_view_t * browser_view) Line 87	C++
 	cefclient.exe!CefWindowCToCpp::ShowAsBrowserModalDialog(scoped_refptr<CefBrowserView> browser_view) Line 89	C++
 	cefclient.exe!client::ViewsWindow::Show() Line 181	C++
 	cefclient.exe!client::ViewsWindow::OnWindowCreated(scoped_refptr<CefWindow> window) Line 738	C++

[magreenblatt]

The |browser_view_layout_->dialog_top_y_| value is configured in BrowserViewLayout::LayoutBookmarkAndInfoBars.

Running as: cefclient --enable-chrome-runtime --url=https://tests/dialogs --hide-frame --hide-controls

The |top| and |browser_view_y| arguments are both 0, so |dialog_top_y_| becomes -3 (kConstrainedWindowOverlap value)

>	libcef.dll!BrowserViewLayout::LayoutBookmarkAndInfoBars(int top, int browser_view_y) Line 610	C++
 	libcef.dll!BrowserViewLayout::Layout(views::View * browser_view) Line 408	C++
 	ui_views.dll!views::View::Layout() Line 873	C++
 	libcef.dll!BrowserView::Layout() Line 4249	C++
 	libcef.dll!CefViewView<BrowserView,CefBrowserViewDelegate>::Layout() Line 167	C++
 	libcef.dll!BrowserView::UpdateUIForContents(content::WebContents * contents) Line 4680	C++
 	libcef.dll!BrowserView::OnActiveTabChanged(content::WebContents * old_contents, content::WebContents * new_contents, int index, int reason) Line 1711	C++
 	libcef.dll!Browser::OnActiveTabChanged(content::WebContents * old_contents, content::WebContents * new_contents, int index, int reason) Line 2674	C++
 	libcef.dll!Browser::OnTabStripModelChanged(TabStripModel * tab_strip_model, const TabStripModelChange & change, const TabStripSelectionChange & selection) Line 1282	C++
 	libcef.dll!TabStripModel::OnChange(const TabStripModelChange & change, const TabStripSelectionChange & selection) Line 417	C++
 	libcef.dll!TabStripModel::InsertWebContentsAtImpl(int index, std::__Cr::unique_ptr<content::WebContents,std::__Cr::default_delete<content::WebContents>> contents, int add_types, std::__Cr::optional<tab_groups::TabGroupId> group) Line 1897	C++
 	libcef.dll!TabStripModel::AddWebContents(std::__Cr::unique_ptr<content::WebContents,std::__Cr::default_delete<content::WebContents>> contents, int index, ui::PageTransition transition, int add_types, std::__Cr::optional<tab_groups::TabGroupId> group) Line 993	C++
 	libcef.dll!Navigate(NavigateParams * params) Line 916	C++
 	libcef.dll!chrome::AddAndReturnTabAt(Browser * browser, const GURL & url, int idx, bool foreground, std::__Cr::optional<tab_groups::TabGroupId> group) Line 47	C++
 	libcef.dll!chrome::AddTabAt(Browser * browser, const GURL & url, int idx, bool foreground, std::__Cr::optional<tab_groups::TabGroupId> group) Line 64	C++
 	libcef.dll!ChromeBrowserHostImpl::Create(const CefBrowserCreateParams & params) Line 49	C++
 	libcef.dll!CefBrowserHostBase::Create(CefBrowserCreateParams & create_params) Line 169	C++
 	libcef.dll!CefBrowserViewImpl::OnBrowserViewAdded() Line 224	C++
 	libcef.dll!ChromeBrowserView::AddedToWidget() Line 57	C++

[magreenblatt]

The |top| and |browser_view_y| arguments are both 0

The |top| value passed to LayoutBookmarkAndInfoBars comes from BrowserFrame::GetTopInset. We don't want to change that value because it will offset the whole BrowserView downwards. Instead, we should add a new callback that directly modifies the value assigned to |dialog_top_y_| in LayoutBookmarkAndInfoBars.