Add GN Build file for extensions_browser target.

extensions/extensions.gyp

 # Copyright 2013 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,
   },
   'targets': [
     {
@@ skipping 216 matching lines @@
             'common/api/sockets/sockets_manifest_permission.cc',
             'common/api/sockets/sockets_manifest_permission.h',
             'common/extension_api.cc',
             'common/manifest_handlers/externally_connectable.cc',
             'common/manifest_handlers/externally_connectable.h',
           ],
         }],
       ],
     },
     {
+      # GN version: //extensions/browser
       'target_name': 'extensions_browser',
       'type': 'static_library',
       'dependencies': [
         '../base/base.gyp:base',
         '../base/base.gyp:base_prefs',
         '../components/components.gyp:keyed_service_content',
         '../components/components.gyp:keyed_service_core',
         '../components/components.gyp:pref_registry',
         '../components/components.gyp:usb_service',
         '../content/content.gyp:content_browser',
@@ skipping 565 matching lines @@
       'type': 'static_library',
       'sources': [ 'browser/api/cast_channel/cast_channel.proto' ],
       'variables': {
           'proto_in_dir': 'browser/api/cast_channel',
           'proto_out_dir': 'extensions/browser/api/cast_channel',
       },
       'includes': [ '../build/protoc.gypi' ]
     },
   ]
 }