ui: Cleanup class/struct forward declarations

ui/views/selection_controller_delegate.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_SELECTION_CONTROLLER_DELEGATE_H_
 #define UI_VIEWS_SELECTION_CONTROLLER_DELEGATE_H_
 
 #include "ui/views/views_export.h"
 
-namespace ui {
-class MouseEvent;
-}
-
 namespace views {
 
 // An interface implemented/managed by a view which uses the
 // SelectionController.
 class VIEWS_EXPORT SelectionControllerDelegate {
  public:
   // Returns the associated RenderText instance to be used for selection.
   virtual gfx::RenderText* GetRenderTextForSelectionController() = 0;
 
   // Methods related to properties of the associated view.
@@ skipping 37 matching lines @@
   // the render text instance, it may be called outside of a pointer action.
   virtual void UpdateSelectionClipboard() = 0;
 
  protected:
   virtual ~SelectionControllerDelegate() {}
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_SELECTION_CONTROLLER_DELEGATE_H_