Add a safe cast for RenderSliderThumb

Source/core/rendering/RenderSlider.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010 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
@@ skipping 81 matching lines @@
 {
     return toSliderThumbElement(toElement(node())->userAgentShadowRoot()->getElementById(ShadowElementNames::sliderThumb()));
 }
 
 void RenderSlider::layout()
 {
     // FIXME: Find a way to cascade appearance.
     // http://webkit.org/b/62535
     RenderBox* thumbBox = sliderThumbElement()->renderBox();
     if (thumbBox && thumbBox->isSliderThumb())
-        static_cast<RenderSliderThumb*>(thumbBox)->updateAppearance(style());
+        toRenderSliderThumb(thumbBox)->updateAppearance(style());
 
     RenderFlexibleBox::layout();
 }
 
 bool RenderSlider::inDragMode() const
 {
     return sliderThumbElement()->active();
 }
 
 } // namespace blink