Add toRenderSVGResourceMarker() fuction, and use it.

Source/core/rendering/svg/RenderSVGResourceMarker.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 57 matching lines @@
     // Generates a transformation matrix usable to render marker content. Handles scaling the marker content
     // acording to SVGs markerUnits="strokeWidth" concept, when a strokeWidth value != -1 is passed in.
     AffineTransform markerContentTransformation(const AffineTransform& contentTransformation, const FloatPoint& origin, float strokeWidth = -1) const;
 
     AffineTransform viewportTransform() const;
 
     mutable AffineTransform m_localToParentTransform;
     FloatRect m_viewport;
 };
 
+inline RenderSVGResourceMarker* toRenderSVGResourceMarker(RenderSVGResource* resource)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!resource || resource->resourceType() == MarkerResourceType);
+    return static_cast<RenderSVGResourceMarker*>(resource);
+}
+
 }
 
 #endif