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

Source/core/svg/SVGFontFaceElement.h

Index: Source/core/svg/SVGFontFaceElement.h
diff --git a/Source/core/svg/SVGFontFaceElement.h b/Source/core/svg/SVGFontFaceElement.h
index 0709eb2632bc9ad236ffd603a4b83c4d8f15828e..35322d528c026fbd1353611426927bf2f457028d 100644
--- a/Source/core/svg/SVGFontFaceElement.h
+++ b/Source/core/svg/SVGFontFaceElement.h
@@ -51,6 +51,8 @@ public:
     
     Result<StyleRuleFontFace> fontFaceRule() const { return m_fontFaceRule; }
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGElement::accept(visitor); }
+
 private:
     SVGFontFaceElement(const QualifiedName&, const Handle<Document>&);