Introduce toSVGUseElement(), use it

Source/core/dom/EventRetargeter.h

 /*
  * Copyright (C) 2013 Google 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 67 matching lines @@
 
     if (!referenceNode->isSVGElement() || !referenceNode->isInShadowTree())
         return referenceNode;
 
     // Spec: The event handling for the non-exposed tree works as if the referenced element had been textually included
     // as a deeply cloned child of the 'use' element, except that events are dispatched to the SVGElementInstance objects
     Element* shadowHostElement = toShadowRoot(referenceNode->treeScope()->rootNode())->host();
     // At this time, SVG nodes are not supported in non-<use> shadow trees.
     if (!shadowHostElement || !shadowHostElement->hasTagName(SVGNames::useTag))
         return referenceNode;
-    SVGUseElement* useElement = static_cast<SVGUseElement*>(shadowHostElement);
+    SVGUseElement* useElement = toSVGUseElement(shadowHostElement);
     if (SVGElementInstance* instance = useElement->instanceForShadowTreeElement(referenceNode))
         return instance;
 
     return referenceNode;
 }
 
 }
 
 #endif // EventRetargeter_h