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

Source/core/svg/properties/SVGPropertyTearOff.cpp

 /*
  * Copyright (C) 2013 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 22 matching lines @@
 
 #include "core/svg/SVGElement.h"
 
 namespace WebCore {
 
 void SVGPropertyTearOffBase::commitChange()
 {
     ASSERT(!isImmutable());
     if (!contextElement() || isAnimVal())
         return;
-    ASSERT(m_attributeName != nullQName());
+    ASSERT(m_attributeName != QualifiedName::null());
     contextElement()->invalidateSVGAttributes();
     contextElement()->svgAttributeChanged(m_attributeName);
 }
 
 } // namespace WebCore