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

Source/core/svg/SVGMaskElement.h

Index: Source/core/svg/SVGMaskElement.h
diff --git a/Source/core/svg/SVGMaskElement.h b/Source/core/svg/SVGMaskElement.h
index 3e041948b6dec873a7ba8ee28acc66255ec9c76b..5e39d545150a230c39ac1553e143bf7c8d5d6ca0 100644
--- a/Source/core/svg/SVGMaskElement.h
+++ b/Source/core/svg/SVGMaskElement.h
@@ -40,6 +40,8 @@ class SVGMaskElement : public SVGStyledElement,
 public:
     static PassRefPtr<SVGMaskElement> create(const QualifiedName&, const Handle<Document>&);
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGStyledElement::accept(visitor); }
+
 private:
     SVGMaskElement(const QualifiedName&, const Handle<Document>&);