Introduce toSVGMaskElement(), and use it

Source/core/rendering/svg/RenderSVGResourceMasker.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 31 matching lines @@
     RenderSVGResourceMasker(SVGMaskElement*);
     virtual ~RenderSVGResourceMasker();
 
     virtual const char* renderName() const { return "RenderSVGResourceMasker"; }
 
     virtual void removeAllClientsFromCache(bool markForInvalidation = true);
     virtual void removeClientFromCache(RenderObject*, bool markForInvalidation = true);
     virtual bool applyResource(RenderObject*, RenderStyle*, GraphicsContext*&, unsigned short resourceMode);
     virtual FloatRect resourceBoundingBox(RenderObject*);
 
-    SVGUnitTypes::SVGUnitType maskUnits() const { return static_cast<SVGMaskElement*>(node())->maskUnitsCurrentValue(); }
-    SVGUnitTypes::SVGUnitType maskContentUnits() const { return static_cast<SVGMaskElement*>(node())->maskContentUnitsCurrentValue(); }
+    SVGUnitTypes::SVGUnitType maskUnits() const { return toSVGMaskElement(node())->maskUnitsCurrentValue(); }
+    SVGUnitTypes::SVGUnitType maskContentUnits() const { return toSVGMaskElement(node())->maskContentUnitsCurrentValue(); }
 
     virtual RenderSVGResourceType resourceType() const { return s_resourceType; }
     static RenderSVGResourceType s_resourceType;
 
 private:
     bool drawContentIntoMaskImage(MaskerData*, ColorSpace, const SVGMaskElement*, RenderObject*);
     void calculateMaskContentRepaintRect();
 
     FloatRect m_maskContentBoundaries;
     HashMap<RenderObject*, MaskerData*> m_masker;
 };
 
 }
 
 #endif