Add toSVGPolylineElement(), and use it

Source/core/rendering/svg/SVGPathData.cpp

 /*
  * Copyright (C) Research In Motion Limited 2011. 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 71 matching lines @@
 
     unsigned size = points.size();
     for (unsigned i = 1; i < size; ++i)
         path.addLineTo(points.at(i));
 
     path.closeSubpath();
 }
 
 static void updatePathFromPolylineElement(SVGElement* element, Path& path)
 {
-    ASSERT(element->hasTagName(SVGNames::polylineTag));
-    SVGPolylineElement* polyline = static_cast<SVGPolylineElement*>(element);
-
-    SVGPointList& points = polyline->pointList();
+    SVGPointList& points = toSVGPolylineElement(element)->pointList();
     if (points.isEmpty())
         return;
 
     path.moveTo(points.first());
 
     unsigned size = points.size();
     for (unsigned i = 1; i < size; ++i)
         path.addLineTo(points.at(i));
 }
 
@@ skipping 43 matching lines @@
         map->set(SVGNames::polygonTag.localName().impl(), updatePathFromPolygonElement);
         map->set(SVGNames::polylineTag.localName().impl(), updatePathFromPolylineElement);
         map->set(SVGNames::rectTag.localName().impl(), updatePathFromRectElement);
     }
 
     if (PathUpdateFunction pathUpdateFunction = map->get(element->localName().impl()))
         (*pathUpdateFunction)(element, path);
 }
 
 } // namespace WebCore