Updated SVGListPropertyHelper as per SVG2 Spec

Source/core/svg/SVGPathSegCurvetoCubicRel.h

Index: Source/core/svg/SVGPathSegCurvetoCubicRel.h
diff --git a/Source/core/svg/SVGPathSegCurvetoCubicRel.h b/Source/core/svg/SVGPathSegCurvetoCubicRel.h
index ebbfda7b5b3e38b341487fea1e6ead19f4589c4d..523c6b6771110ec884fd22ffc6b3ec6837c8c6ba 100644
--- a/Source/core/svg/SVGPathSegCurvetoCubicRel.h
+++ b/Source/core/svg/SVGPathSegCurvetoCubicRel.h
@@ -34,6 +34,11 @@ public:
         return adoptRefWillBeNoop(new SVGPathSegCurvetoCubicRel(element, x, y, x1, y1, x2, y2));
     }
 
+    virtual PassRefPtrWillBeRawPtr<SVGPathSeg> clone() override
+    {
+        return adoptRefWillBeNoop(new SVGPathSegCurvetoCubicRel(nullptr, x(), y(), x1(), y1(), x2(), y2()));
+    }
+
 private:
     SVGPathSegCurvetoCubicRel(SVGPathElement* element, float x, float y, float x1, float y1, float x2, float y2)
         : SVGPathSegCurvetoCubic(element, x, y, x1, y1, x2, y2) { }