Add toSVGPolylineElement(), and use it

Source/core/svg/SVGPolylineElement.h

Index: Source/core/svg/SVGPolylineElement.h
diff --git a/Source/core/svg/SVGPolylineElement.h b/Source/core/svg/SVGPolylineElement.h
index 3f64854e663330eb533fe9186bdf672e66fe1d48..57f948c9a5b567d3253226db56dbb15d2f40a22d 100644
--- a/Source/core/svg/SVGPolylineElement.h
+++ b/Source/core/svg/SVGPolylineElement.h
@@ -21,6 +21,7 @@
 #ifndef SVGPolylineElement_h
 #define SVGPolylineElement_h
 
+#include "SVGNames.h"
 #include "core/svg/SVGPolyElement.h"
 
 namespace WebCore {
@@ -33,6 +34,12 @@ private:
     SVGPolylineElement(const QualifiedName&, Document*);
 };
 
+inline SVGPolylineElement* toSVGPolylineElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(SVGNames::polylineTag));
+    return static_cast<SVGPolylineElement*>(node);
+}
+
 } // namespace WebCore
 
 #endif