platform-window: Add a PlatformWindowFactory for creating windows.

ui/platform_window/x11/x11_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,
     'use_x11': 1,
   },
   'targets': [{
     'target_name': 'x11_window',
     'type': '<(component)',
     'dependencies': [
       '../../../base/base.gyp:base',
       '../../../build/linux/system.gyp:x11',
       '../../../skia/skia.gyp:skia',
       '../../events/events.gyp:events',
       '../../events/platform/x11/x11_events_platform.gyp:x11_events_platform',
       '../../gfx/x/gfx_x11.gyp:gfx_x11',
       '../platform_window.gyp:platform_window',
     ],
     'defines': [ 'X11_WINDOW_IMPLEMENTATION' ],
     'sources': [
       'x11_window.cc',
       'x11_window.h',
       'x11_window_export.h',
+      'x11_window_factory.cc',
+      'x11_window_factory.h',
     ],
   }],
 }