Remove attrNodeListMap.

Source/core/dom/Attr.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)
  * Copyright (C) 2004, 2005, 2006, 2007, 2009, 2010, 2012 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 201 matching lines @@
     m_element = nullptr;
 }
 
 void Attr::attachToElement(Element* element, const AtomicString& attachedLocalName)
 {
     ASSERT(!m_element);
     m_element = element;
     m_standaloneValueOrAttachedLocalName = attachedLocalName;
 }
 
-void Attr::clearWeakMembers(Visitor* visitor)
-{
-    if (m_element && !visitor->isAlive(m_element))
-        detachFromElementWithValue(value());
-}
-
 void Attr::trace(Visitor* visitor)
 {
-    visitor->registerWeakMembers<Attr, &Attr::clearWeakMembers>(this);
+    visitor->trace(m_element);
     ContainerNode::trace(visitor);
 }
 
 }