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

Source/core/svg/SVGPathSegListTearOff.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 42 matching lines @@
         RefPtr<SVGPathSeg> value = passValue;
         value->setContextElement(contextElement);
         return value.release();
     }
 };
 
 class SVGPathSegListTearOff FINAL :
     public SVGListPropertyTearOffHelper<SVGPathSegListTearOff, SVGPathSegList>,
     public ScriptWrappable {
 public:
-    static PassRefPtr<SVGPathSegListTearOff> create(PassRefPtr<SVGPathSegList> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = nullQName())
+    static PassRefPtr<SVGPathSegListTearOff> create(PassRefPtr<SVGPathSegList> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = QualifiedName::null())
     {
         return adoptRef(new SVGPathSegListTearOff(target, contextElement, propertyIsAnimVal, attributeName));
     }
 
 private:
-    SVGPathSegListTearOff(PassRefPtr<SVGPathSegList> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = nullQName())
+    SVGPathSegListTearOff(PassRefPtr<SVGPathSegList> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = QualifiedName::null())
         : SVGListPropertyTearOffHelper<SVGPathSegListTearOff, SVGPathSegList>(target, contextElement, propertyIsAnimVal, attributeName)
     {
         ScriptWrappable::init(this);
     }
 };
 
 } // namespace WebCore
 
 #endif // SVGPathSegListTearOff_h_