Add widevine and extensions resources to GN build.

extensions/extensions_strings.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.
 
 {
   'targets': [
     {
+      # GN version: //extensions:strings
       'target_name': 'extensions_strings',
       'type': 'none',
       'actions': [
         {
           'action_name': 'generate_extensions_strings',
           'variables': {
             'grit_grd_file': 'extensions_strings.grd',
             'grit_out_dir': '<(SHARED_INTERMEDIATE_DIR)/extensions/strings',
           },
           'includes': [ '../build/grit_action.gypi' ],
         },
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           '<(SHARED_INTERMEDIATE_DIR)/extensions/strings',
         ],
       },
       # This target generates extensions_strings.h so it must be built before
       # targets that depend on it.
       'hard_dependency': 1,
     },
   ],
 }