ui: Cleanup class/struct forward declarations

ui/views/mus/drop_target_mus.h

 // Copyright 2016 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_VIEWS_MUS_DROP_TARGET_MUS_H_
 #define UI_VIEWS_MUS_DROP_TARGET_MUS_H_
 
 #include <map>
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "base/macros.h"
 #include "services/ui/public/cpp/window_drop_target.h"
 #include "ui/aura/window_observer.h"
 
 namespace aura {
-class RootWindow;
 class Window;
 
 namespace client {
 class DragDropDelegate;
 }
 }
 
 namespace ui {
 class DropTargetEvent;
 class OSExchangeData;
@@ skipping 55 matching lines @@
   // and cache it so we don't send this multiple times. We reset this value on
   // leave or drop.
   std::unique_ptr<ui::OSExchangeData> os_exchange_data_;
 
   DISALLOW_COPY_AND_ASSIGN(DropTargetMus);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_MUS_DROP_TARGET_MUS_H_