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

Source/core/svg/SVGMetadataElement.h

Index: Source/core/svg/SVGMetadataElement.h
diff --git a/Source/core/svg/SVGMetadataElement.h b/Source/core/svg/SVGMetadataElement.h
index 385a8ee254ffa25bbec2083d61c36f8fe537b46b..a4622c515c8ef5e911532ffbe00bc1e48d11a925 100644
--- a/Source/core/svg/SVGMetadataElement.h
+++ b/Source/core/svg/SVGMetadataElement.h
@@ -30,6 +30,8 @@ class SVGMetadataElement : public SVGElement {
 public:
     static PassRefPtr<SVGMetadataElement> create(const QualifiedName&, const Handle<Document>&);
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGElement::accept(visitor); }
+
 private:
     SVGMetadataElement(const QualifiedName&, const Handle<Document>&);
 };