Reduce forward declarations in core/rendering

Source/core/rendering/RenderSlider.h

 /*
  * Copyright (C) 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  *
  */
 
 #ifndef RenderSlider_h
 #define RenderSlider_h
 
 #include "core/rendering/RenderFlexibleBox.h"
 
 namespace WebCore {
 
 class HTMLInputElement;
-class MouseEvent;
 class SliderThumbElement;
 
 class RenderSlider FINAL : public RenderFlexibleBox {
 public:
     static const int defaultTrackLength;
 
     explicit RenderSlider(HTMLInputElement*);
     virtual ~RenderSlider();
 
     bool inDragMode() const;
 
 private:
     virtual const char* renderName() const OVERRIDE { return "RenderSlider"; }
     virtual bool isSlider() const OVERRIDE { return true; }
 
     virtual int baselinePosition(FontBaseline, bool firstLine, LineDirectionMode, LinePositionMode = PositionOnContainingLine) const OVERRIDE;
     virtual void computeIntrinsicLogicalWidths(LayoutUnit& minLogicalWidth, LayoutUnit& maxLogicalWidth) const OVERRIDE;
     virtual void computePreferredLogicalWidths() OVERRIDE;
     virtual void layout() OVERRIDE;
 
     SliderThumbElement* sliderThumbElement() const;
 };
 
 DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderSlider, isSlider());
 
 } // namespace WebCore
 
 #endif // RenderSlider_h