aura: Use PlatformWindow from WindowTreeHost.

ui/platform_window/win/win_window.h

 // 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.
 
 #ifndef UI_PLATFORM_WINDOW_WIN_WIN_WINDOW_H_
 #define UI_PLATFORM_WINDOW_WIN_WIN_WINDOW_H_
 
 #include "base/compiler_specific.h"
 #include "ui/gfx/win/window_impl.h"
 #include "ui/platform_window/platform_window.h"
@@ skipping 54 matching lines @@
   LRESULT OnCreate(CREATESTRUCT* create_struct);
   void OnDestroy();
   void OnPaint(HDC);
   void OnWindowPosChanged(WINDOWPOS* window_pos);
 
   PlatformWindowDelegate* delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(WinWindow);
 };
 
+namespace test {
+
+// Set true to let WindowTreeHostWin use a popup window
+// with no frame/title so that the window size and test's
+// expectations matches.
+WIN_WINDOW_EXPORT void SetUsePopupAsRootWindowForTest(bool use);
+
+}  // namespace test
+
 }  // namespace ui
 
 #endif  // UI_PLATFORM_WINDOW_WIN_WIN_WINDOW_H_