ui: Cleanup class/struct forward declarations

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 "base/macros.h"
 #include "ui/gfx/win/window_impl.h"
 #include "ui/platform_window/platform_window.h"
 #include "ui/platform_window/win/win_window_export.h"
 
 namespace ui {
 
+class PlatformWindowDelegate;
+
 class WIN_WINDOW_EXPORT WinWindow : public NON_EXPORTED_BASE(PlatformWindow),
                                     public gfx::WindowImpl {
  public:
   WinWindow(PlatformWindowDelegate* delegate, const gfx::Rect& bounds);
   ~WinWindow() override;
 
  private:
   void Destroy();
 
   // PlatformWindow:
@@ skipping 57 matching lines @@
 // 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_