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

Source/core/svg/SVGTextContentElement.h

Index: Source/core/svg/SVGTextContentElement.h
diff --git a/Source/core/svg/SVGTextContentElement.h b/Source/core/svg/SVGTextContentElement.h
index adcc76f2dfc491c1347d2735adf0a28e68cde0f6..693a15c2f638d9edc6be6265f5b73209c42a3091 100644
--- a/Source/core/svg/SVGTextContentElement.h
+++ b/Source/core/svg/SVGTextContentElement.h
@@ -97,6 +97,8 @@ public:
     PassRefPtr<SVGAnimatedLength> textLengthAnimated();
     static const SVGPropertyInfo* textLengthPropertyInfo();
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGStyledElement::accept(visitor); }
+
 protected:
     SVGTextContentElement(const QualifiedName&, const Handle<Document>&);