Add toSVGPolylineElement(), 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 f6ccd9523ce72a78c2b3440c489679dca27e378d..d9085d668814f183a656f59db645d798afe2f30b 100644
--- a/Source/core/rendering/svg/SVGPathData.cpp
+++ b/Source/core/rendering/svg/SVGPathData.cpp
@@ -89,10 +89,7 @@ static void updatePathFromPolygonElement(SVGElement* element, Path& path)
 
 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;