Introduce toSVGGradientElement(), use it

Source/core/svg/SVGRadialGradientElement.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2008 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007 Rob Buis <buis@kde.org>
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008 Dirk Schulze <krit@webkit.org>
  * Copyright (C) Research In Motion Limited 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
@@ skipping 167 matching lines @@
 
             if (!attributes.hasFr() && current->hasAttribute(SVGNames::frAttr))
                 attributes.setFr(radial->frCurrentValue());
         }
 
         processedGradients.add(current);
 
         // Respect xlink:href, take attributes from referenced element
         Node* refNode = SVGURIReference::targetElementFromIRIString(current->hrefCurrentValue(), document());
         if (refNode && (refNode->hasTagName(SVGNames::radialGradientTag) || refNode->hasTagName(SVGNames::linearGradientTag))) {
-            current = static_cast<SVGGradientElement*>(refNode);
+            current = toSVGGradientElement(refNode);
 
             // Cycle detection
             if (processedGradients.contains(current)) {
                 current = 0;
                 break;
             }
 
             isRadial = current->hasTagName(SVGNames::radialGradientTag);
         } else
             current = 0;
@@ skipping 12 matching lines @@
 {
     return cxCurrentValue().isRelative()
         || cyCurrentValue().isRelative()
         || rCurrentValue().isRelative()
         || fxCurrentValue().isRelative()
         || fyCurrentValue().isRelative()
         || frCurrentValue().isRelative();
 }
 
 }