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

Source/core/svg/SVGAnimatedEnumerationBase.cpp

 /*
  * 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 37 matching lines @@
         exceptionState.throwDOMException(NoModificationAllowedError, "The attribute is read-only.");
         return;
     }
 
     baseValue()->setValue(value, exceptionState);
     if (exceptionState.hadException())
         return;
 
     m_baseValueUpdated = true;
 
-    ASSERT(this->attributeName() != nullQName());
+    ASSERT(this->attributeName() != QualifiedName::null());
     contextElement()->invalidateSVGAttributes();
     contextElement()->svgAttributeChanged(this->attributeName());
 }
 
 }