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

ash/ash.gyp

 # 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.
 
 {
   'variables': {
     'chromium_code': 1,
     'grit_out_dir': '<(SHARED_INTERMEDIATE_DIR)/chrome',
   },
 
@@ skipping 91 matching lines @@
         'monitor/monitor_controller.h',
         'monitor/multi_monitor_manager.cc',
         'monitor/multi_monitor_manager.h',
         'monitor/secondary_monitor_view.cc',
         'monitor/secondary_monitor_view.h',
         'screen_ash.cc',
         'screen_ash.h',
         'screenshot_delegate.h',
         'shell.cc',
         'shell.h',
+        'shell_context_menu.cc',
+        'shell_context_menu.h',
         'shell_delegate.h',
         'shell_factory.h',
         'shell_window_ids.h',
         'status_area/status_area_view.cc',
         'status_area/status_area_view.h',
         'system/audio/audio_observer.h',
         'system/audio/tray_volume.cc',
         'system/audio/tray_volume.h',
         'system/bluetooth/bluetooth_observer.h',
         'system/bluetooth/tray_bluetooth.cc',
@@ skipping 355 matching lines @@
             'shell/cocoa/app-Info.plist',
           ],
           'xcode_settings': {
             'INFOPLIST_FILE': 'shell/cocoa/app-Info.plist',
           },
         }],
       ],
     },
   ],
 }