Use low resolution wallpaper for small screens.

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..813e338266e25e08d919be8a2ecc502359f3908d 100644
--- a/ash/desktop_background/desktop_background_controller.h
+++ b/ash/desktop_background/desktop_background_controller.h
@@ -71,8 +71,10 @@ class ASH_EXPORT DesktopBackgroundController {
   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.