ui: Cleanup class/struct forward declarations

ui/aura/window_tree_host_mac.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_AURA_WINDOW_TREE_HOST_MAC_H_
 #define UI_AURA_WINDOW_TREE_HOST_MAC_H_
 
 #include <vector>
 
 #include "base/mac/scoped_nsobject.h"
 #include "base/macros.h"
 #include "ui/aura/aura_export.h"
 #include "ui/aura/window_tree_host.h"
 #include "ui/gfx/geometry/insets.h"
 #include "ui/gfx/geometry/rect.h"
 
-namespace ui {
-class MouseEvent;
-}
-
 namespace aura {
 
 class AURA_EXPORT WindowTreeHostMac : public WindowTreeHost {
  public:
   explicit WindowTreeHostMac(const gfx::Rect& bounds);
   ~WindowTreeHostMac() override;
 
  private:
   // WindowTreeHost Overrides.
   ui::EventSource* GetEventSource() override;
@@ skipping 17 matching lines @@
 
  private:
   base::scoped_nsobject<NSWindow> window_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowTreeHostMac);
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_WINDOW_TREE_HOST_MAC_H_