ui: Cleanup class/struct forward declarations

ui/base/clipboard/clipboard_win.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_BASE_CLIPBOARD_CLIPBOARD_WIN_H_
 #define UI_BASE_CLIPBOARD_CLIPBOARD_WIN_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <memory>
 
 #include "base/macros.h"
 #include "ui/base/clipboard/clipboard.h"
 
+namespace base {
+namespace win {
+class MessageWindow;
+}
+}
+
+namespace gfx {
+class Size;
+}
+
 namespace ui {
 
 class ClipboardWin : public Clipboard {
  private:
   friend class Clipboard;
 
   ClipboardWin();
   ~ClipboardWin() override;
 
   // Clipboard overrides:
@@ skipping 46 matching lines @@
 
   // Mark this as mutable so const methods can still do lazy initialization.
   mutable std::unique_ptr<base::win::MessageWindow> clipboard_owner_;
 
   DISALLOW_COPY_AND_ASSIGN(ClipboardWin);
 };
 
 }  // namespace ui
 
 #endif  // UI_BASE_CLIPBOARD_CLIPBOARD_WIN_H_