Adding a gray semi transparent backdrop behind the topmost window within the default container

ash/wm/overview/window_overview.cc

Index: ash/wm/overview/window_overview.cc
diff --git a/ash/wm/overview/window_overview.cc b/ash/wm/overview/window_overview.cc
index 8b1408fdfa4f12a7cda8348683a4feafb20552c7..8f0cdf3512dbd995daae248397fc575f209d9ea9 100644
--- a/ash/wm/overview/window_overview.cc
+++ b/ash/wm/overview/window_overview.cc
@@ -145,6 +145,7 @@ WindowOverview::WindowOverview(WindowSelector* window_selector,
   ash::Shell::GetInstance()->PrependPreTargetHandler(this);
   Shell* shell = Shell::GetInstance();
   shell->metrics()->RecordUserMetricsAction(UMA_WINDOW_OVERVIEW);
+  shell->OnOverviewModeStarted();
   HideAndTrackNonOverviewWindows();
 }
 
@@ -163,10 +164,12 @@ WindowOverview::~WindowOverview() {
   }
   if (cursor_client_)
     cursor_client_->UnlockCursor();
-  ash::Shell::GetInstance()->RemovePreTargetHandler(this);
+  Shell* shell = Shell::GetInstance();
+  shell->RemovePreTargetHandler(this);
   UMA_HISTOGRAM_MEDIUM_TIMES(
       "Ash.WindowSelector.TimeInOverview",
       base::Time::Now() - overview_start_time_);
+  shell->OnOverviewModeEnded();

[sky, 2014/03/06 01:03:18]

How about moving this to its own patch?

[Mr4D (OOO till 08-26), 2014/03/06 17:17:16]

It is already. Please look at the latest rev of the patch from yesterday
morning. :)

 }
 
 void WindowOverview::SetSelection(size_t index) {