Disable window occlusion APIs when running browser tests

The positioning of windows is not predictable when running tests, and
some tests can end up having their renderer processes backgrounded
due to occlusion, causing unpredictable behavior.

Disable occlusion API use when running tests. Occlusion is a Mac-only
API for the moment, but leave this as non-Mac so that it can be picked
up if needed.

Also, get rid of the "we're on 10.9+" check. We always are now.

BUG=558585

Review URL: https://codereview.chromium.org/1762883002

Cr-Commit-Position: refs/heads/master@{#379500}

content/browser/web_contents/web_contents_view_mac.mm

@@ -8,6 +8,7 @@
 
 #include <string>
 
+#include "base/command_line.h"
 #import "base/mac/mac_util.h"
 #import "base/mac/scoped_sending_event.h"
 #include "base/mac/sdk_forward_declarations.h"
@@ -23,6 +24,7 @@
 #include "content/common/view_messages.h"
 #include "content/public/browser/web_contents_delegate.h"
 #include "content/public/browser/web_contents_view_delegate.h"
+#include "content/public/common/content_switches.h"
 #include "skia/ext/skia_utils_mac.h"
 #import "third_party/mozilla/NSPasteboard+Utils.h"
 #include "ui/base/clipboard/custom_data_helper.h"
@@ -636,10 +638,12 @@ - (void)viewWillMoveToWindow:(NSWindow*)newWindow {
   NSNotificationCenter* notificationCenter =
       [NSNotificationCenter defaultCenter];
 
-  // Occlusion notification APIs are new in Mavericks.
-  bool supportsOcclusionAPIs = base::mac::IsOSMavericksOrLater();
+  // Occlusion is highly undesirable for browser tests, since it will
+  // flakily change test behavior.
+  static bool isDisabled = base::CommandLine::ForCurrentProcess()->HasSwitch(
+      switches::kDisableBackgroundingOccludedWindowsForTesting);
 
-  if (supportsOcclusionAPIs) {
+  if (!isDisabled) {
     if (oldWindow) {
       [notificationCenter
           removeObserver:self

content/public/common/content_switches.cc

@@ -90,6 +90,11 @@ const char kDisableAcceleratedVideoDecode[] =
 // users with many windows/tabs and lots of memory.
 const char kDisableBackingStoreLimit[]      = "disable-backing-store-limit";
 
+// Disable backgrounding renders for occluded windows. Done for tests to avoid
+// nondeterministic behavior.
+extern const char kDisableBackgroundingOccludedWindowsForTesting[] =
+    "disable-backgrounding-occluded-windows";
+
 // Disable one or more Blink runtime-enabled features.
 // Use names from RuntimeEnabledFeatures.in, separated by commas.
 // Applied after kEnableBlinkFeatures, and after other flags that change these

content/public/common/content_switches.h

@@ -35,6 +35,8 @@ CONTENT_EXPORT extern const char kDisableAcceleratedJpegDecoding[];
 CONTENT_EXPORT extern const char kDisableAcceleratedMjpegDecode[];
 CONTENT_EXPORT extern const char kDisableAcceleratedVideoDecode[];
 extern const char kDisableBackingStoreLimit[];
+CONTENT_EXPORT extern const char
+    kDisableBackgroundingOccludedWindowsForTesting[];
 CONTENT_EXPORT extern const char kDisableBlinkFeatures[];
 CONTENT_EXPORT extern const char kDisableDatabases[];
 CONTENT_EXPORT extern const char kDisableDelayAgnosticAec[];

content/public/test/browser_test_base.cc

@@ -200,6 +200,11 @@ void BrowserTestBase::SetUp() {
   if (use_software_compositing_)
     command_line->AppendSwitch(switches::kDisableGpu);
 
+  // The layout of windows on screen is unpredictable during tests, so disable
+  // occlusion when running browser tests.
+  base::CommandLine::ForCurrentProcess()->AppendSwitch(
+      switches::kDisableBackgroundingOccludedWindowsForTesting);
+
 #if defined(USE_AURA)
   // Most tests do not need pixel output, so we don't produce any. The command
   // line can override this behaviour to allow for visual debugging.