[oilpan]: Avoid refcounting QualifiedName's nullQName when tracing.

Source/core/svg/SVGPointTearOff.h

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 23 matching lines @@
 #include "bindings/v8/ScriptWrappable.h"
 #include "core/svg/SVGPoint.h"
 #include "core/svg/properties/SVGPropertyTearOff.h"
 
 namespace WebCore {
 
 class SVGMatrixTearOff;
 
 class SVGPointTearOff : public SVGPropertyTearOff<SVGPoint>, public ScriptWrappable {
 public:
-    static PassRefPtr<SVGPointTearOff> create(PassRefPtr<SVGPoint> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = nullQName())
+    static PassRefPtr<SVGPointTearOff> create(PassRefPtr<SVGPoint> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = QualifiedName::null())
     {
         return adoptRef(new SVGPointTearOff(target, contextElement, propertyIsAnimVal, attributeName));
     }
 
     void setX(float, ExceptionState&);
     void setY(float, ExceptionState&);
     float x() { return target()->x(); }
     float y() { return target()->y(); }
 
     PassRefPtr<SVGPointTearOff> matrixTransform(PassRefPtr<SVGMatrixTearOff>);
 
 protected:
-    SVGPointTearOff(PassRefPtr<SVGPoint>, SVGElement* contextElement, PropertyIsAnimValType, const QualifiedName& attributeName = nullQName());
+    SVGPointTearOff(PassRefPtr<SVGPoint>, SVGElement* contextElement, PropertyIsAnimValType, const QualifiedName& attributeName = QualifiedName::null());
 };
 
 } // namespace WebCore
 
 #endif // SVGPointTearOff_h_