Add GN Build file for extensions_common 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': [
     {
+      # GN version: //extensions/common
       'target_name': 'extensions_common',
       'type': 'static_library',
       'dependencies': [
         # TODO(benwells): figure out what to do with the api target and
         # api resources compiled into the chrome resource bundle.
         # http://crbug.com/162530
         '../chrome/chrome_resources.gyp:chrome_resources',
         '../components/components.gyp:url_matcher',
         '../content/content.gyp:content_common',
         '../crypto/crypto.gyp:crypto',
@@ skipping 721 matching lines @@
               '<(SHARED_INTERMEDIATE_DIR)/extensions/extensions_renderer_resources.pak',
             ],
             'pak_output': '<(PRODUCT_DIR)/extensions_unittests_resources.pak',
           },
           'includes': [ '../build/repack_action.gypi' ],
         },
       ],
     },
   ]
 }