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

Source/core/svg/SVGTRefElement.h

Index: Source/core/svg/SVGTRefElement.h
diff --git a/Source/core/svg/SVGTRefElement.h b/Source/core/svg/SVGTRefElement.h
index 6dfac523a1789173dbb86c12d6f781a57e27e619..ae8189ab2893ec320efb34cbffdf09b3437cf09a 100644
--- a/Source/core/svg/SVGTRefElement.h
+++ b/Source/core/svg/SVGTRefElement.h
@@ -35,6 +35,8 @@ class SVGTRefElement : public SVGTextPositioningElement,
 public:
     static PassRefPtr<SVGTRefElement> create(const QualifiedName&, const Handle<Document>&);
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGTextPositioningElement::accept(visitor); }
+
 private:
     friend class SVGTRefTargetEventListener;