Remove "set wallpaper..." option in system context menu when guest logged in

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 a368443984d6c687e5dade3787aea9be2e7a525a..828ba6b2da080df6352f9188990724220a335e08 100644
--- a/ash/desktop_background/desktop_background_controller.h
+++ b/ash/desktop_background/desktop_background_controller.h
@@ -23,6 +23,10 @@ class UserWallpaperDelegate {
 
   // Open the set wallpaper page in the browser.
   virtual void OpenSetWallpaperPage() = 0;
+
+  // Returns true if user can open set wallpaper page. Only guest user returns
+  // false currently.
+  virtual bool CanOpenSetWallpaperPage() = 0;
 };
 
 // A class to listen for login and desktop background change events and set the