Add browser_extensions to GN build.

components/onc.gypi

 # 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.
 
 {
   'targets': [
     {
+      # GN version: //components/onc
       'target_name': 'onc_component',
       'type': '<(component)',
       'dependencies': [
         '../base/base.gyp:base',
       ],
       'include_dirs': [
         '..',
       ],
       'defines': [
         'ONC_IMPLEMENTATION',
       ],
       'sources': [
         'onc/onc_constants.cc',
         'onc/onc_constants.h',
         'onc/onc_export.h',
       ],
     },
   ],
 }