Get rid of the huge delay in background resize during rotation

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 1b6d7b509113a16d5dc944165dca3ba1ba9f7bef..ba865fd44e2c15a445990f1852f8fcb583e7e59f 100644
--- a/ash/desktop_background/desktop_background_controller.h
+++ b/ash/desktop_background/desktop_background_controller.h
@@ -133,8 +133,9 @@ class ASH_EXPORT DesktopBackgroundController
   // Send notification that background animation finished.
   void NotifyAnimationFinished();
 
-  // Reload the wallpaper.
-  void UpdateWallpaper();
+  // Reload the wallpaper. |clear_cache| specifies whether to clear the
+  // wallpaper cahce or not.
+  void UpdateWallpaper(bool clear_cache);
 
   void set_wallpaper_reload_delay_for_test(bool value) {
     wallpaper_reload_delay_ = value;