Introduce toSVGPolygonElement(), and use it

Source/core/rendering/svg/SVGPathData.cpp

Index: Source/core/rendering/svg/SVGPathData.cpp
diff --git a/Source/core/rendering/svg/SVGPathData.cpp b/Source/core/rendering/svg/SVGPathData.cpp
index 23fc96525eb6acd6607001a1a40fc9c2d0980c7e..f6ccd9523ce72a78c2b3440c489679dca27e378d 100644
--- a/Source/core/rendering/svg/SVGPathData.cpp
+++ b/Source/core/rendering/svg/SVGPathData.cpp
@@ -74,10 +74,7 @@ static void updatePathFromPathElement(SVGElement* element, Path& path)
 
 static void updatePathFromPolygonElement(SVGElement* element, Path& path)
 {
-    ASSERT(element->hasTagName(SVGNames::polygonTag));
-    SVGPolygonElement* polygon = static_cast<SVGPolygonElement*>(element);
-
-    SVGPointList& points = polygon->pointList();
+    SVGPointList& points = toSVGPolygonElement(element)->pointList();
     if (points.isEmpty())
         return;