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

Source/core/svg/SVGFEMorphologyElement.h

Index: Source/core/svg/SVGFEMorphologyElement.h
diff --git a/Source/core/svg/SVGFEMorphologyElement.h b/Source/core/svg/SVGFEMorphologyElement.h
index 4a9e3ed39dad4d76db34e706f650b3f6b26f1671..14ca2c136fa3c48604034b8150c7fe75ad163657 100644
--- a/Source/core/svg/SVGFEMorphologyElement.h
+++ b/Source/core/svg/SVGFEMorphologyElement.h
@@ -63,6 +63,8 @@ public:
 
     void setRadius(float radiusX, float radiusY);
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGFilterPrimitiveStandardAttributes::accept(visitor); }
+
 private:
     SVGFEMorphologyElement(const QualifiedName&, const Handle<Document>&);