Introduce toSVGGradientElement(), use it

Source/core/svg/SVGLinearGradientElement.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 150 matching lines @@
 
             if (!attributes.hasY2() && current->hasAttribute(SVGNames::y2Attr))
                 attributes.setY2(linear->y2CurrentValue());
         }
 
         processedGradients.add(current);
 
         // Respect xlink:href, take attributes from referenced element
         Node* refNode = SVGURIReference::targetElementFromIRIString(current->hrefCurrentValue(), document());
         if (refNode && (refNode->hasTagName(SVGNames::linearGradientTag) || refNode->hasTagName(SVGNames::radialGradientTag))) {
-            current = static_cast<SVGGradientElement*>(refNode);
+            current = toSVGGradientElement(refNode);
 
             // Cycle detection
             if (processedGradients.contains(current)) {
                 current = 0;
                 break;
             }
 
             isLinear = current->hasTagName(SVGNames::linearGradientTag);
         } else
             current = 0;
     }
 
     return true;
 }
 
 bool SVGLinearGradientElement::selfHasRelativeLengths() const
 {
     return x1CurrentValue().isRelative()
         || y1CurrentValue().isRelative()
         || x2CurrentValue().isRelative()
         || y2CurrentValue().isRelative();
 }
 
 }