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

Source/core/svg/SVGTextPositioningElement.h

Index: Source/core/svg/SVGTextPositioningElement.h
diff --git a/Source/core/svg/SVGTextPositioningElement.h b/Source/core/svg/SVGTextPositioningElement.h
index 43706e9fb5a63cc3c057864f714fc648f7625373..fa2a1076dae4e61532d3ce9bb49e9f6af03f3c21 100644
--- a/Source/core/svg/SVGTextPositioningElement.h
+++ b/Source/core/svg/SVGTextPositioningElement.h
@@ -32,6 +32,8 @@ class SVGTextPositioningElement : public SVGTextContentElement {
 public:
     static SVGTextPositioningElement* elementFromRenderer(RenderObject*);
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGTextContentElement::accept(visitor); }
+
 protected:
     SVGTextPositioningElement(const QualifiedName&, const Handle<Document>&);