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

ash/shell.cc

 // 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.
 
 #include "ash/shell.h"
 
 #include <algorithm>
 
 #include "ash/app_list/app_list.h"
 #include "ash/ash_switches.h"
@@ skipping 234 matching lines @@
 
   virtual ~DummyUserWallpaperDelegate() {}
 
   virtual const int GetUserWallpaperIndex() OVERRIDE {
     return 0;
   }
 
   virtual void OpenSetWallpaperPage() OVERRIDE {
   }
 
+  virtual bool IsLoggedInAsGuest() OVERRIDE {
+    return false;
+  }
+
  private:
    DISALLOW_COPY_AND_ASSIGN(DummyUserWallpaperDelegate);
 };
 
 class DummySystemTrayDelegate : public SystemTrayDelegate {
  public:
   DummySystemTrayDelegate()
       : muted_(false),
         wifi_enabled_(true),
         cellular_enabled_(true),
@@ skipping 659 matching lines @@
   desktop_background_controller_->SetDesktopBackgroundImageMode(
       GetWallpaper(index), GetWallpaperInfo(index).layout);
 }
 
 void Shell::DisableWorkspaceGridLayout() {
   if (workspace_controller_.get())
     workspace_controller_->workspace_manager()->set_grid_size(0);
 }
 
 }  // namespace ash