Use low resolution wallpaper for small screens.

ash/desktop_background/desktop_background_controller.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_DESKTOP_BACKGROUND_DESKTOP_BACKGROUND_CONTROLLER_H_
 #define ASH_DESKTOP_BACKGROUND_DESKTOP_BACKGROUND_CONTROLLER_H_
 
 #include "ash/ash_export.h"
 #include "ash/desktop_background/desktop_background_resources.h"
 #include "ash/wm/window_animations.h"
@@ skipping 53 matching lines @@
   WallpaperLayout GetWallpaperLayout() const;
 
   // Provides current image on the background, or empty SkBitmap if there is
   // no image, e.g. background is solid color.
   SkBitmap GetCurrentWallpaperImage();
 
   // Initialize root window's background.
   void OnRootWindowAdded(aura::RootWindow* root_window);
 
   // Loads default wallpaper at |index| asynchronously and sets to current
-  // wallpaper after loaded.
-  void SetDefaultWallpaper(int index);
+  // wallpaper after loaded. When |force_reload| is true, reload wallpaper
+  // for all root windows even if |index| is the same as current wallpaper. It
+  // must be true when a different resolution of current wallpaper is needed.
+  void SetDefaultWallpaper(int index, bool force_reload);
 
   // Sets the user selected custom wallpaper. Called when user selected a file
   // from file system or changed the layout of wallpaper.
   void SetCustomWallpaper(const gfx::ImageSkia& wallpaper,
                           WallpaperLayout layout);
 
   // Cancels the current wallpaper loading operation.
   void CancelPendingWallpaperOperation();
 
   // Sets the desktop background to solid color mode and creates a solid
@@ skipping 37 matching lines @@
   scoped_refptr<WallpaperOperation> wallpaper_op_;
 
   base::WeakPtrFactory<DesktopBackgroundController> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(DesktopBackgroundController);
 };
 
 }  // namespace ash
 
 #endif  // ASH_DESKTOP_BACKGROUND_DESKTOP_BACKGROUND_CONTROLLER_H_