Fix regression introduced with r159483. The RenderScrollbar's parent class had the signature of a …

Source/core/rendering/RenderScrollbar.h

 /*
  * Copyright (C) 2008, 2009 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 42 matching lines @@
 
     IntRect buttonRect(ScrollbarPart);
     IntRect trackRect(int startLength, int endLength);
     IntRect trackPieceRectWithMargins(ScrollbarPart, const IntRect&);
 
     int minimumThumbLength();
 
     virtual bool isOverlayScrollbar() const { return false; }
 
 private:
-    virtual void setParent(ScrollView*);
+    virtual void setParent(Widget*) OVERRIDE;
     virtual void setEnabled(bool);
 
     virtual void paint(GraphicsContext*, const IntRect& damageRect);
 
     virtual void setHoveredPart(ScrollbarPart);
     virtual void setPressedPart(ScrollbarPart);
 
     virtual void styleChanged();
 
     virtual bool isCustomScrollbar() const { return true; }
@@ skipping 18 matching lines @@
     ASSERT_WITH_SECURITY_IMPLICATION(!scrollbar || scrollbar->isCustomScrollbar());
     return static_cast<RenderScrollbar*>(scrollbar);
 }
 
 // This will catch anyone doing an unnecessary cast.
 void toRenderScrollbar(const RenderScrollbar*);
 
 } // namespace WebCore
 
 #endif // RenderScrollbar_h