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

Source/core/svg/properties/SVGAnimatedProperty.h

 /*
  * 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.
 G*     * Redistributions in binary form must reproduce the above
@@ skipping 190 matching lines @@
     void setBaseVal(PrimitiveType value, WebCore::ExceptionState& exceptionState)
     {
         if (this->isReadOnly()) {
             exceptionState.throwDOMException(NoModificationAllowedError, "The attribute is read-only.");
             return;
         }
 
         this->baseValue()->setValue(value);
         m_baseValueUpdated = true;
 
-        ASSERT(this->attributeName() != nullQName());
+        ASSERT(this->attributeName() != QualifiedName::null());
         this->contextElement()->invalidateSVGAttributes();
         this->contextElement()->svgAttributeChanged(this->attributeName());
     }
 
     PrimitiveType animVal()
     {
         return this->currentValue()->value();
     }
 
 protected:
@@ skipping 78 matching lines @@
     // When animated:
     //     m_animValTearOff targets m_currentValue.
     //     m_baseValTearOff targets m_baseValue.
     RefPtr<TearOffType> m_baseValTearOff;
     RefPtr<TearOffType> m_animValTearOff;
 };
 
 }
 
 #endif // SVGAnimatedProperty_h