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

Source/core/svg/SVGSVGElement.h

Index: Source/core/svg/SVGSVGElement.h
diff --git a/Source/core/svg/SVGSVGElement.h b/Source/core/svg/SVGSVGElement.h
index 407e661c0993047821c866b900995897ef984a2d..fe276b867a7bbd4ef1a3acfc575bf82557bb3c3f 100644
--- a/Source/core/svg/SVGSVGElement.h
+++ b/Source/core/svg/SVGSVGElement.h
@@ -138,6 +138,8 @@ public:
 
     bool hasEmptyViewBox() const { return viewBoxIsValid() && viewBox().isEmpty(); }
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGStyledTransformableElement::accept(visitor); }
+
 private:
     SVGSVGElement(const QualifiedName&, const Handle<Document>&);
     virtual ~SVGSVGElement();