Remove root_window_ member from DesktopBackgroundController.

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 3a90684955263293bd2ac737bda0c65add733d8d..ea243ed42b25c81824907f05c479c9514aab8318 100644
--- a/ash/desktop_background/desktop_background_controller.h
+++ b/ash/desktop_background/desktop_background_controller.h
@@ -46,7 +46,7 @@ class ASH_EXPORT DesktopBackgroundController {
     BACKGROUND_SOLID_COLOR
   };
 
-  explicit DesktopBackgroundController(aura::RootWindow* root_window);
+  DesktopBackgroundController();
   virtual ~DesktopBackgroundController();
 
   // Gets the desktop background mode.
@@ -91,8 +91,6 @@ class ASH_EXPORT DesktopBackgroundController {
   // Called after wallpaper loaded successfully.
   void OnWallpaperLoadCompleted(scoped_refptr<WallpaperOperation> wo);
 
-  aura::RootWindow* root_window_;
-
   // Can change at runtime.
   BackgroundMode desktop_background_mode_;