aura: Unify WindowTreeHost for some platforms

ui/platform_window/platform_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': [{
     # GN version: //ui/platform_window
     'target_name': 'platform_window',
     'type': 'none',
     'sources': [
       'platform_window.h',
+      'platform_window.cc',
       'platform_window_delegate.h',
     ],
   }],
 }