platform-window: Add a PlatformWindowFactory for creating windows.

ui/platform_window/win/win_window.gyp

 # Copyright 2014 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': [{
     'target_name': 'win_window',
     'type': '<(component)',
     'dependencies': [
       '../../../base/base.gyp:base',
       '../../../skia/skia.gyp:skia',
       '../../events/events.gyp:events',
       '../../gfx/gfx.gyp:gfx',
       '../../gfx/gfx.gyp:gfx_geometry',
       '../platform_window.gyp:platform_window',
     ],
     'defines': [ 'WIN_WINDOW_IMPLEMENTATION' ],
     'sources': [
       'win_window.cc',
       'win_window.h',
       'win_window_export.h',
+      'win_window_factory.cc',
+      'win_window_factory.h',
     ],
   }],
 }