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

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index 1d58b39dbf3ff1df76f573635627dd24d0348ba0..566117762c99302528b9838f377aee5ed41a3c5f 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -291,6 +291,12 @@ class ASH_EXPORT Shell
   // Called when a casting session is started or stopped.
   void OnCastingSessionStartedOrStopped(bool started);
 
+  // Called when the overview mode has started.
+  void OnOverviewModeStarted();
+
+  // Called when the overview mode has ended.
+  void OnOverviewModeEnded();
+
   // Initializes |shelf_|.  Does nothing if it's already initialized.
   void CreateShelf();
 
@@ -311,6 +317,9 @@ class ASH_EXPORT Shell
   // Turn the always maximize mode window manager on or off.
   void EnableMaximizeModeWindowManager(bool enable);
 
+  // Test if the MaximizeModeWindowManager is enabled or not.
+  bool IsMaximizeModeWindowManagerEnabled();
+
   keyboard::KeyboardController* keyboard_controller() {
     return keyboard_controller_.get();
   }