Introduce toSVGPolygonElement(), and use it

Source/core/svg/SVGPolygonElement.h

Index: Source/core/svg/SVGPolygonElement.h
diff --git a/Source/core/svg/SVGPolygonElement.h b/Source/core/svg/SVGPolygonElement.h
index 5a2bd1dc92e5e8ab3a1d4c1e84f042aaa4c6eb8d..dfa85d7576678797a9e1dab92f851086285a6ec8 100644
--- a/Source/core/svg/SVGPolygonElement.h
+++ b/Source/core/svg/SVGPolygonElement.h
@@ -21,6 +21,7 @@
 #ifndef SVGPolygonElement_h
 #define SVGPolygonElement_h
 
+#include "SVGNames.h"
 #include "core/svg/SVGPolyElement.h"
 
 namespace WebCore {
@@ -33,6 +34,12 @@ private:
     SVGPolygonElement(const QualifiedName&, Document*);
 };
 
+inline SVGPolygonElement* toSVGPolygonElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(SVGNames::polygonTag));
+    return static_cast<SVGPolygonElement*>(node);
+}
+
 } // namespace WebCore
 
 #endif