ui: Cleanup class/struct forward declarations

ui/aura/mus/window_tree_client_observer.h

 // Copyright 2015 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_AURA_MUS_WINDOW_TREE_CLIENT_OBSERVER_H_
 #define UI_AURA_MUS_WINDOW_TREE_CLIENT_OBSERVER_H_
 
 #include "ui/aura/aura_export.h"
 
 namespace aura {
 
-class Window;
 class WindowTreeClient;
 
 class AURA_EXPORT WindowTreeClientObserver {
  public:
   // Called early on in the destructor of WindowTreeClient; before any windows
   // have been destroyed.
   virtual void OnWillDestroyClient(WindowTreeClient* client) {}
 
   // Called at the end of WindowTreeClient's destructor. At this point
   // observers should drop all references to |client|.
   virtual void OnDidDestroyClient(WindowTreeClient* client) {}
 
  protected:
   virtual ~WindowTreeClientObserver() {}
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_MUS_WINDOW_TREE_CLIENT_OBSERVER_H_