Add toRenderSVGResourceRadialGradient() fuction, and use it.

Source/core/rendering/svg/RenderSVGResourceRadialGradient.h

 /*
  * Copyright (C) 2006 Nikolas Zimmermann <zimmermann@kde.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.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 34 matching lines @@
 
     FloatPoint centerPoint(const RadialGradientAttributes&) const;
     FloatPoint focalPoint(const RadialGradientAttributes&) const;
     float radius(const RadialGradientAttributes&) const;
     float focalRadius(const RadialGradientAttributes&) const;
 
 private:
     RadialGradientAttributes m_attributes;
 };
 
+inline RenderSVGResourceRadialGradient* toRenderSVGResourceRadialGradient(RenderSVGResourceContainer* resource)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!resource || resource->resourceType() == RadialGradientResourceType);
+    return static_cast<RenderSVGResourceRadialGradient*>(resource);
+}
+
 }
 
 #endif