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

Source/core/svg/SVGAnimationElement.h

Index: Source/core/svg/SVGAnimationElement.h
diff --git a/Source/core/svg/SVGAnimationElement.h b/Source/core/svg/SVGAnimationElement.h
index 18e70bbf870450db1435bb7febb102c0cb66ebca..e53104d0a3323497fc39e908b1e9827f3843a0be 100644
--- a/Source/core/svg/SVGAnimationElement.h
+++ b/Source/core/svg/SVGAnimationElement.h
@@ -76,6 +76,8 @@ public:
     float getCurrentTime() const;
     float getSimpleDuration(ExceptionCode&) const;
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGSMILElement::accept(visitor); }
+
     // ElementTimeControl
     virtual void beginElement();
     virtual void beginElementAt(float offset);