Preload default wallpaper.

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 b575654bdd60a65231d974e51d94eb54d41cd78e..91a486c0080a2c38d13e08c12a8414ac651aec22 100644
--- a/ash/desktop_background/desktop_background_controller.h
+++ b/ash/desktop_background/desktop_background_controller.h
@@ -70,6 +70,11 @@ class ASH_EXPORT DesktopBackgroundController {
   // Initialize root window's background.
   void OnRootWindowAdded(aura::RootWindow* root_window);
 
+  // Loads default wallpaper at |index| asynchronously but does not set the
+  // loaded image to current wallpaper. Resource bundle will cache the loaded
+  // image.
+  void TriggerCacheDefaultWallpaper(int index);
+
   // Loads default wallpaper at |index| asynchronously and sets to current
   // wallpaper after loaded.
   void SetDefaultWallpaper(int index);