Add toRenderSVGResourceMasker() fuction, and use it.

Source/core/rendering/svg/RenderSVGResourceMasker.h

 /*
  * Copyright (C) Research In Motion Limited 2009-2010. 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,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 41 matching lines @@
     virtual RenderSVGResourceType resourceType() const { return s_resourceType; }
     static RenderSVGResourceType s_resourceType;
 
 private:
     void calculateMaskContentRepaintRect();
     void drawMaskContent(GraphicsContext*, const FloatRect& targetBoundingBox);
 
     FloatRect m_maskContentBoundaries;
 };
 
+inline RenderSVGResourceMasker* toRenderSVGResourceMasker(RenderSVGResource* resource)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!resource || resource->resourceType() == MaskerResourceType);
+    return static_cast<RenderSVGResourceMasker*>(resource);
+}
+
 }
 
 #endif