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

ash/shell_observer.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_SHELL_OBSERVER_H_
 #define ASH_SHELL_OBSERVER_H_
 
 #include "ash/ash_export.h"
 #include "ash/system/user/login_status.h"
 
@@ skipping 21 matching lines @@
   // Invoked when the shelf alignment in |root_window| is changed.
   virtual void OnShelfAlignmentChanged(aura::Window* root_window) {}
 
   // Invoked when the projection touch HUD is toggled.
   virtual void OnTouchHudProjectionToggled(bool enabled) {}
 
   // Invoked when entering or exiting fullscreen mode in |root_window|.
   virtual void OnFullscreenStateChanged(bool is_fullscreen,
                                         aura::Window* root_window) {}
 
+  // The overview mode has started.
+  virtual void OnOverviewModeStarted() {}
+
+  // The overview mode has ended.
+  virtual void OnOverviewModeEnded() {}
+
  protected:
   virtual ~ShellObserver() {}
 };
 
 }  // namespace ash
 
 #endif  // ASH_SHELL_OBSERVER_H_