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

Source/core/dom/Element.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Peter Kelly (pmk@post.com)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012, 2013 Apple Inc. All rights reserved.
  *           (C) 2007 Eric Seidel (eric@webkit.org)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 1019 matching lines @@
         cache->handleAttributeChanged(name, this);
 }
 
 bool Element::hasLegalLinkAttribute(const QualifiedName&) const
 {
     return false;
 }
 
 const QualifiedName& Element::subResourceAttributeName() const
 {
-    return nullQName();
+    return QualifiedName::null();
 }
 
 inline void Element::attributeChangedFromParserOrByCloning(const QualifiedName& name, const AtomicString& newValue, AttributeModificationReason reason)
 {
     if (name == isAttr)
         CustomElementRegistrationContext::setTypeExtension(this, newValue);
     attributeChanged(name, newValue, reason);
 }
 
 template <typename CharacterType>
@@ skipping 2272 matching lines @@
 
 void Element::trace(Visitor* visitor)
 {
     if (hasRareData())
         visitor->trace(elementRareData());
 
     ContainerNode::trace(visitor);
 }
 
 } // namespace WebCore