Forget about DesktopBackgroundWidget if it was deleted

ash/desktop_background/desktop_background_controller.h

Index: ash/desktop_background/desktop_background_controller.h
diff --git a/ash/desktop_background/desktop_background_controller.h b/ash/desktop_background/desktop_background_controller.h
index 0251c1c8d4e84af333c2458976a2847cf56f85b9..71eb8ff917af0f767aea3b01691fbcd53e8a43c3 100644
--- a/ash/desktop_background/desktop_background_controller.h
+++ b/ash/desktop_background/desktop_background_controller.h
@@ -125,6 +125,10 @@ class ASH_EXPORT DesktopBackgroundController : public aura::WindowObserver {
   // Move all desktop widgets to unlocked container.
   void MoveDesktopToUnlockedContainer();
 
+  // Drop references to background view for |root_window|, because the view
+  // was deleted.
+  void CleanupView(aura::RootWindow* root_window);
+
   // WindowObserver implementation.
   virtual void OnWindowDestroying(aura::Window* window) OVERRIDE;