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

Source/core/svg/SVGGlyphRefElement.h

Index: Source/core/svg/SVGGlyphRefElement.h
diff --git a/Source/core/svg/SVGGlyphRefElement.h b/Source/core/svg/SVGGlyphRefElement.h
index 12b218f0613f0db572718d9955072bd21fba9461..da4988ca0d70e7f7b3932d40245473ec405bad8b 100644
--- a/Source/core/svg/SVGGlyphRefElement.h
+++ b/Source/core/svg/SVGGlyphRefElement.h
@@ -46,6 +46,8 @@ public:
     float dy() const { return m_dy; }
     void setDy(float, ExceptionCode&);
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGStyledElement::accept(visitor); }
+
 private:
     SVGGlyphRefElement(const QualifiedName&, const Handle<Document>&);