[oilpan] Implement accept methods to trace the class hierarchy of SVGElement and all sub-classes.

Source/core/svg/SVGMPathElement.h

Index: Source/core/svg/SVGMPathElement.h
diff --git a/Source/core/svg/SVGMPathElement.h b/Source/core/svg/SVGMPathElement.h
index df54a5a9f55f10b921722013fb7aaf2bb0411ef3..d912de2bbf0fde6489dcabfd2e4cb6748c576ff0 100644
--- a/Source/core/svg/SVGMPathElement.h
+++ b/Source/core/svg/SVGMPathElement.h
@@ -43,6 +43,8 @@ public:
 
     void targetPathChanged();
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGElement::accept(visitor); }
+
 private:
     SVGMPathElement(const QualifiedName&, const Handle<Document>&);