Add toSVGClipPathElement(), and use it

Source/core/rendering/svg/RenderSVGResourceClipper.h

 /*
  * Copyright (C) Research In Motion Limited 2009-2010. 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 34 matching lines @@
     // clipPath can be clipped too, but don't have a boundingBox or repaintRect. So we can't call
     // applyResource directly and use the rects from the object, since they are empty for RenderSVGResources
     // FIXME: We made applyClippingToContext public because we cannot call applyResource on HTML elements (it asserts on RenderObject::objectBoundingBox)
     bool applyClippingToContext(RenderObject*, const FloatRect&, const FloatRect&, GraphicsContext*);
     virtual FloatRect resourceBoundingBox(RenderObject*);
 
     virtual RenderSVGResourceType resourceType() const { return ClipperResourceType; }
 
     bool hitTestClipContent(const FloatRect&, const FloatPoint&);
 
-    SVGUnitTypes::SVGUnitType clipPathUnits() const { return static_cast<SVGClipPathElement*>(node())->clipPathUnitsCurrentValue(); }
+    SVGUnitTypes::SVGUnitType clipPathUnits() const { return toSVGClipPathElement(node())->clipPathUnitsCurrentValue(); }
 
     static RenderSVGResourceType s_resourceType;
 private:
     bool pathOnlyClipping(GraphicsContext*, const AffineTransform&, const FloatRect&);
     bool drawContentIntoMaskImage(ClipperData*, const FloatRect& objectBoundingBox);
     void calculateClipContentRepaintRect();
 
     FloatRect m_clipBoundaries;
     HashMap<RenderObject*, ClipperData*> m_clipper;
 };
 
 }
 
 #endif