Cherry-picking into M41 branch 2272.

Update guest zoom when it differs from the embedder's.

In some cases, for example deleting a zoom level via Content Settings,
the generated ZoomChangeEventData does not know the old zoom level and
just sets it to be the same as the new level, even though it did change.

GuestViewBase::OnZoomChanged() should therefore compare the new zoom
level against the guest's zoom level when deciding if it should early-
out.

BUG=447542

Review URL: https://codereview.chromium.org/819613003

Cr-Commit-Position: refs/heads/master@{#311722}
(cherry picked from commit 4b1d3a8)

Review URL: https://codereview.chromium.org/800913006

Cr-Commit-Position: refs/branch-heads/2272@{#39}
Cr-Branched-From: 827a380-refs/heads/master@{#310958}

extensions/browser/guest_view/guest_view_base.cc

@@ -530,12 +530,15 @@ GuestViewBase::~GuestViewBase() {
 
 void GuestViewBase::OnZoomChanged(
     const ui_zoom::ZoomController::ZoomChangedEventData& data) {
-  if (content::ZoomValuesEqual(data.old_zoom_level, data.new_zoom_level))
+  auto guest_zoom_controller =
+      ui_zoom::ZoomController::FromWebContents(web_contents());
+  if (content::ZoomValuesEqual(data.new_zoom_level,
+                               guest_zoom_controller->GetZoomLevel())) {
     return;
-  // When the embedder's zoom level is changed, then we also update the
+  }
+  // When the embedder's zoom level doesn't match the guest's, then update the
   // guest's zoom level to match.
-  ui_zoom::ZoomController::FromWebContents(web_contents())
-      ->SetZoomLevel(data.new_zoom_level);
+  guest_zoom_controller->SetZoomLevel(data.new_zoom_level);
 }
 
 void GuestViewBase::DispatchEventToEmbedder(Event* event) {