ui: Cleanup class/struct forward declarations

ui/wm/public/activation_delegate.h

 // Copyright (c) 2012 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_WM_PUBLIC_ACTIVATION_DELEGATE_H_
 #define UI_WM_PUBLIC_ACTIVATION_DELEGATE_H_
 
 #include "ui/aura/aura_export.h"
 
-namespace ui {
-class Event;
-}
-
 namespace aura {
 class Window;
 namespace client {
 
 // An interface implemented by an object that configures and responds to changes
 // to a window's activation state.
 class AURA_EXPORT ActivationDelegate {
  public:
   // Returns true if the window should be activated.
   virtual bool ShouldActivate() const = 0;
 
  protected:
   virtual ~ActivationDelegate() {}
 };
 
 // Sets/Gets the ActivationDelegate on the Window. No ownership changes.
 AURA_EXPORT void SetActivationDelegate(Window* window,
                                        ActivationDelegate* delegate);
 AURA_EXPORT ActivationDelegate* GetActivationDelegate(Window* window);
 
 }  // namespace client
 }  // namespace aura
 
 #endif  // UI_WM_PUBLIC_ACTIVATION_DELEGATE_H_