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

Source/core/svg/properties/SVGPropertyTearOff.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.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 68 matching lines @@
     }
 
     const QualifiedName& attributeName()
     {
         return m_attributeName;
     }
 
     virtual AnimatedPropertyType type() const = 0;
 
 protected:
-    SVGPropertyTearOffBase(SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = nullQName())
+    SVGPropertyTearOffBase(SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = QualifiedName::null())
         : m_contextElement(contextElement)
         , m_propertyIsAnimVal(propertyIsAnimVal)
         , m_isReadOnlyProperty(false)
         , m_attributeName(attributeName)
     {
     }
 
 private:
     // These references are kept alive from V8 wrapper to prevent reference cycles
     SVGElement* m_contextElement;
@@ skipping 15 matching lines @@
     {
         m_target = target;
     }
 
     virtual AnimatedPropertyType type() const OVERRIDE
     {
         return Property::classType();
     }
 
 protected:
-    SVGPropertyTearOff(PassRefPtr<Property> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = nullQName())
+    SVGPropertyTearOff(PassRefPtr<Property> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = QualifiedName::null())
         : SVGPropertyTearOffBase(contextElement, propertyIsAnimVal, attributeName)
         , m_target(target)
     {
         ASSERT(m_target);
     }
 
 private:
     RefPtr<Property> m_target;
 };
 
 }
 
 #endif // SVGPropertyTearOff_h