Add support code to test the cast system tray item.

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,
     # These files lists are shared with the GN build.
     'ash_sources': [
       'accelerators/accelerator_commands.cc',
@@ skipping 268 matching lines @@
       'system/audio/tray_audio.h',
       'system/audio/tray_audio_delegate.h',
       'system/audio/volume_view.cc',
       'system/audio/volume_view.h',
       'system/bluetooth/bluetooth_observer.h',
       'system/bluetooth/tray_bluetooth.cc',
       'system/bluetooth/tray_bluetooth.h',
       'system/brightness_control_delegate.h',
       'system/cast/tray_cast.cc',
       'system/cast/tray_cast.h',
+      'system/cast/tray_cast_testing_interface.h',
       'system/chromeos/audio/audio_detailed_view.cc',
       'system/chromeos/audio/audio_detailed_view.h',
       'system/chromeos/audio/tray_audio_chromeos.cc',
       'system/chromeos/audio/tray_audio_chromeos.h',
       'system/chromeos/audio/tray_audio_delegate_chromeos.cc',
       'system/chromeos/audio/tray_audio_delegate_chromeos.h',
       'system/chromeos/bluetooth/bluetooth_notification_controller.cc',
       'system/chromeos/bluetooth/bluetooth_notification_controller.h',
       'system/chromeos/brightness/brightness_controller_chromeos.cc',
       'system/chromeos/brightness/brightness_controller_chromeos.h',
@@ skipping 1052 matching lines @@
                   '../tools/xdisplaycheck/xdisplaycheck.gyp:xdisplaycheck',
                 ],
               }
             ],
           ],
         },
       ],
     }],
   ],
 }