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

Source/core/svg/SVGNumberTearOff.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/SVGNumber.h"
 #include "core/svg/properties/SVGPropertyTearOff.h"
 
 namespace WebCore {
 
 class SVGMatrix;
 
 class SVGNumberTearOff : public SVGPropertyTearOff<SVGNumber>, public ScriptWrappable {
 public:
-    static PassRefPtr<SVGNumberTearOff> create(PassRefPtr<SVGNumber> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = nullQName())
+    static PassRefPtr<SVGNumberTearOff> create(PassRefPtr<SVGNumber> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = QualifiedName::null())
     {
         return adoptRef(new SVGNumberTearOff(target, contextElement, propertyIsAnimVal, attributeName));
     }
 
     void setValue(float, ExceptionState&);
     float value() { return target()->value(); }
 
 protected:
-    SVGNumberTearOff(PassRefPtr<SVGNumber>, SVGElement* contextElement, PropertyIsAnimValType, const QualifiedName& attributeName = nullQName());
+    SVGNumberTearOff(PassRefPtr<SVGNumber>, SVGElement* contextElement, PropertyIsAnimValType, const QualifiedName& attributeName = QualifiedName::null());
 };
 
 } // namespace WebCore
 
 #endif // SVGNumberTearOff_h_