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

Source/core/svg/SVGRadialGradientElement.h

Index: Source/core/svg/SVGRadialGradientElement.h
diff --git a/Source/core/svg/SVGRadialGradientElement.h b/Source/core/svg/SVGRadialGradientElement.h
index 17bf6efa162579e9ffe789ecc6fde87ab629bdab..406e14cfbe97a405d1df733f43d6a1a382566708 100644
--- a/Source/core/svg/SVGRadialGradientElement.h
+++ b/Source/core/svg/SVGRadialGradientElement.h
@@ -36,6 +36,8 @@ public:
 
     bool collectGradientAttributes(RadialGradientAttributes&);
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGGradientElement::accept(visitor); }
+
 private:
     SVGRadialGradientElement(const QualifiedName&, const Handle<Document>&);