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

Source/core/dom/custom/CustomElementRegistry.cpp

 /*
  * Copyright (C) 2012 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:
  *
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 63 matching lines @@
     if (!constructorBuilder->isFeatureAllowed()) {
         CustomElementException::throwException(CustomElementException::CannotRegisterFromExtension, type, exceptionState);
         return 0;
     }
 
     if (!CustomElement::isValidName(type, validNames)) {
         CustomElementException::throwException(CustomElementException::InvalidName, type, exceptionState);
         return 0;
     }
 
-    QualifiedName tagName = nullQName();
+    QualifiedName tagName = QualifiedName::null();
     if (!constructorBuilder->validateOptions(type, tagName, exceptionState))
         return 0;
 
     ASSERT(tagName.namespaceURI() == HTMLNames::xhtmlNamespaceURI || tagName.namespaceURI() == SVGNames::svgNamespaceURI);
 
     // FIXME: This should be done earlier in validateOptions.
     if (m_registeredTypeNames.contains(type)) {
         CustomElementException::throwException(CustomElementException::TypeAlreadyRegistered, type, exceptionState);
         return 0;
     }
@@ skipping 25 matching lines @@
 
     return definition.get();
 }
 
 CustomElementDefinition* CustomElementRegistry::find(const CustomElementDescriptor& descriptor) const
 {
     return m_definitions.get(descriptor);
 }
 
 } // namespace WebCore