Make webkit_user_agent a standalone gyp

webkit/support/webkit_support.gyp

 # Copyright (c) 2012 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.
 
 {
-  'includes': [
-    '../common/user_agent/webkit_user_agent.gypi',
-  ],
   'conditions': [
     # TODO(stuartmorgan): All dependencies from code built on iOS to
     # webkit/ should be removed, at which point this condition can be
     # removed.
     ['OS != "ios"', {
       'includes': [
         '../../build/win_precompile.gypi',
         '../appcache/webkit_appcache.gypi',
         '../base/webkit_base.gypi',
         '../glue/webkit_glue.gypi',
         '../plugins/webkit_plugins.gypi',
         '../renderer/media/webkit_media.gypi',
         '../storage/webkit_storage.gypi',
         'webkit_support.gypi',
       ],
     }],
   ],
 }