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

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_
 #pragma once
 
 #include "ash/ash_export.h"
 #include "ash/desktop_background/desktop_background_resources.h"
 #include "base/basictypes.h"
 
 class SkBitmap;
 
 namespace ash {
 
 class UserWallpaperDelegate {
  public:
   virtual ~UserWallpaperDelegate() {}
 
   // Gets the index of user selected wallpaper.
   virtual const int GetUserWallpaperIndex() = 0;
 
   // Open the set wallpaper page in the browser.
   virtual void OpenSetWallpaperPage() = 0;
+
+  // Returns true if we're logged in as a Guest.
+  virtual bool IsLoggedInAsGuest() = 0;
 };
 
 // A class to listen for login and desktop background change events and set the
 // corresponding default wallpaper in Aura shell.
 class ASH_EXPORT DesktopBackgroundController {
  public:
   enum BackgroundMode {
     BACKGROUND_IMAGE,
     BACKGROUND_SOLID_COLOR
   };
@@ skipping 21 matching lines @@
  private:
   // Can change at runtime.
   BackgroundMode desktop_background_mode_;
 
   DISALLOW_COPY_AND_ASSIGN(DesktopBackgroundController);
 };
 
 }  // namespace ash
 
 #endif  // ASH_DESKTOP_BACKGROUND_DESKTOP_BACKGROUND_CONTROLLER_H_