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

ash/shell_observer.h

Index: ash/shell_observer.h
diff --git a/ash/shell_observer.h b/ash/shell_observer.h
index 9d1db021e0c8b33624953537b9307ad10fbab6a9..1c0cc45be8409ae5c765ed0e0d017dea34c90368 100644
--- a/ash/shell_observer.h
+++ b/ash/shell_observer.h
@@ -39,6 +39,12 @@ class ASH_EXPORT ShellObserver {
   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() {}
 };