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

Source/core/svg/animation/SVGSMILElement.h

Index: Source/core/svg/animation/SVGSMILElement.h
diff --git a/Source/core/svg/animation/SVGSMILElement.h b/Source/core/svg/animation/SVGSMILElement.h
index 2c1f601b7c8c00b6eb41c3481dd4b58e199e6a6e..f6e3d25e257eae55bb8365157fc801d23b098382 100644
--- a/Source/core/svg/animation/SVGSMILElement.h
+++ b/Source/core/svg/animation/SVGSMILElement.h
@@ -111,6 +111,8 @@ public:
     virtual void clearAnimatedType(SVGElement* targetElement) = 0;
     virtual void applyResultsToTarget() = 0;
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGElement::accept(visitor); }
+
 protected:
     void addBeginTime(SMILTime eventTime, SMILTime endTime, SMILTimeWithOrigin::Origin = SMILTimeWithOrigin::ParserOrigin);
     void addEndTime(SMILTime eventTime, SMILTime endTime, SMILTimeWithOrigin::Origin = SMILTimeWithOrigin::ParserOrigin);