[Chromoting] Add GN build support for ar_sample_app target.

remoting/app_remoting_webapp.gyp

 # Copyright 2014 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.
 
 {
   'includes': [
     '../remoting/remoting_locales.gypi',
     '../remoting/app_remoting_webapp_build.gypi',
   ],
 
@@ skipping 74 matching lines @@
               '<@(remoting_webapp_js_proto_files)',
             ],
           },
         ],  # actions
       }],
     ],  # conditions
   },
 
   'targets': [
     {
+      # GN version: //remoting/webapp:ar_sample_app
       # Sample AppRemoting app.
       'target_name': 'ar_sample_app',
       'app_key': 'Sample_App',
       'app_id': 'ljacajndfccfgnfohlgkdphmbnpkjflk',
       'app_client_id': 'sample_client_id',
       'app_name': 'App Remoting Client',
       'app_description': 'App Remoting client',
       'app_capabilities': ['GOOGLE_DRIVE'],
       'manifest_key': 'remotingdevbuild',
     },
   ],  # end of targets
 }