Introduce toSVGRadialGradientElement(), and use it

Source/core/rendering/svg/RenderSVGResourceGradient.cpp

 /*
  * Copyright (C) 2006 Nikolas Zimmermann <zimmermann@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
  * version 2 of the License, or (at your option) any later version.
@@ skipping 44 matching lines @@
 {
     ASSERT(object);
     ASSERT(style);
     ASSERT(context);
     ASSERT(resourceMode != ApplyToDefaultMode);
 
     // Be sure to synchronize all SVG properties on the gradientElement _before_ processing any further.
     // Otherwhise the call to collectGradientAttributes() in createTileImage(), may cause the SVG DOM property
     // synchronization to kick in, which causes removeAllClientsFromCache() to be called, which in turn deletes our
     // GradientData object! Leaving out the line below will cause svg/dynamic-updates/SVG*GradientElement-svgdom* to crash.
-    SVGGradientElement* gradientElement = static_cast<SVGGradientElement*>(node());
+    SVGGradientElement* gradientElement = toSVGGradientElement(node());
     if (!gradientElement)
         return false;
 
     if (m_shouldCollectGradientAttributes) {
         gradientElement->synchronizeAnimatedSVGAttribute(anyQName());
         if (!collectGradientAttributes(gradientElement))
             return false;
 
         m_shouldCollectGradientAttributes = false;
     }
@@ skipping 102 matching lines @@
         return SpreadMethodReflect;
     case SVGSpreadMethodRepeat:
         return SpreadMethodRepeat;
     }
 
     ASSERT_NOT_REACHED();
     return SpreadMethodPad;
 }
 
 }