Add toSVGClipPathElement(), and use it

Source/core/svg/SVGClipPathElement.h

Index: Source/core/svg/SVGClipPathElement.h
diff --git a/Source/core/svg/SVGClipPathElement.h b/Source/core/svg/SVGClipPathElement.h
index c5bbb882a8ce8566049f440509c799363037dc73..5667aa7dd16d24d4fe3c989d6345e247abaa8217 100644
--- a/Source/core/svg/SVGClipPathElement.h
+++ b/Source/core/svg/SVGClipPathElement.h
@@ -21,6 +21,7 @@
 #ifndef SVGClipPathElement_h
 #define SVGClipPathElement_h
 
+#include "SVGNames.h"
 #include "core/svg/SVGAnimatedBoolean.h"
 #include "core/svg/SVGAnimatedEnumeration.h"
 #include "core/svg/SVGExternalResourcesRequired.h"
@@ -55,6 +56,12 @@ private:
     END_DECLARE_ANIMATED_PROPERTIES
 };
 
+inline SVGClipPathElement* toSVGClipPathElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(SVGNames::clipPathTag));
+    return static_cast<SVGClipPathElement*>(node);
+}
+
 }
 
 #endif