Add a user agent stylesheet for extension options pages

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 729 matching lines @@
         'renderer/render_view_observer_natives.h',
         'renderer/request_sender.cc',
         'renderer/request_sender.h',
         'renderer/resource_bundle_source_map.cc',
         'renderer/resource_bundle_source_map.h',
         'renderer/resources/app_runtime_custom_bindings.js',
         'renderer/resources/binding.js',
         'renderer/resources/context_menus_custom_bindings.js',
         'renderer/resources/entry_id_manager.js',
         'renderer/resources/event.js',
+        'renderer/resources/extension.css',
         'renderer/resources/extension_custom_bindings.js',
+        'renderer/resources/extension_fonts.css',
         'renderer/resources/greasemonkey_api.js',
         'renderer/resources/i18n_custom_bindings.js',
         'renderer/resources/image_util.js',
         'renderer/resources/json_schema.js',
         'renderer/resources/last_error.js',
         'renderer/resources/messaging.js',
         'renderer/resources/messaging_utils.js',
         'renderer/resources/permissions_custom_bindings.js',
         'renderer/resources/platform_app.css',
         'renderer/resources/platform_app.js',
@@ skipping 286 matching lines @@
           'browser/api/cast_channel/logging.proto'
       ],
       'variables': {
           'proto_in_dir': 'browser/api/cast_channel',
           'proto_out_dir': 'extensions/browser/api/cast_channel',
       },
       'includes': [ '../build/protoc.gypi' ]
     },
   ]
 }