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

Source/core/svg/SVGPolygonElement.h

Index: Source/core/svg/SVGPolygonElement.h
diff --git a/Source/core/svg/SVGPolygonElement.h b/Source/core/svg/SVGPolygonElement.h
index 9b42207461329d68ccd7021fade80c1c73dab9e9..b464064eef26d67ca40bf75855fe133ac3b55487 100644
--- a/Source/core/svg/SVGPolygonElement.h
+++ b/Source/core/svg/SVGPolygonElement.h
@@ -30,6 +30,8 @@ class SVGPolygonElement : public SVGPolyElement {
 public:
     static PassRefPtr<SVGPolygonElement> create(const QualifiedName&, const Handle<Document>&);
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGPolyElement::accept(visitor); }
+
 private:
     SVGPolygonElement(const QualifiedName&, const Handle<Document>&);
 };