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

Source/core/svg/SVGLinearGradientElement.h

Index: Source/core/svg/SVGLinearGradientElement.h
diff --git a/Source/core/svg/SVGLinearGradientElement.h b/Source/core/svg/SVGLinearGradientElement.h
index ca2c2b5eb020581a736ff9630ff88d4f5b5b8403..49daedbdb83938342a634d7f029f133d83cd965c 100644
--- a/Source/core/svg/SVGLinearGradientElement.h
+++ b/Source/core/svg/SVGLinearGradientElement.h
@@ -36,6 +36,8 @@ public:
 
     bool collectGradientAttributes(LinearGradientAttributes&);
 
+    virtual void accept(Visitor* visitor) const OVERRIDE { SVGGradientElement::accept(visitor); }
+
 private:
     SVGLinearGradientElement(const QualifiedName&, const Handle<Document>&);