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

Source/core/svg/SVGSymbolElement.h

Index: Source/core/svg/SVGSymbolElement.h
diff --git a/Source/core/svg/SVGSymbolElement.h b/Source/core/svg/SVGSymbolElement.h
index 7d7122f648216c93c12790b8adfcc36d59239fc4..d3f4fd4470d61e7d23f91912806737321bc9c9b2 100644
--- a/Source/core/svg/SVGSymbolElement.h
+++ b/Source/core/svg/SVGSymbolElement.h
@@ -40,6 +40,8 @@ class SVGSymbolElement : public SVGStyledElement,
 public:
     static PassRefPtr<SVGSymbolElement> create(const QualifiedName&, const Handle<Document>&);
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGStyledElement::accept(visitor); }
+
 private:
     SVGSymbolElement(const QualifiedName&, const Handle<Document>&);