Oilpan: add missing tracing of Attr objects.

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 29 matching lines @@
     : ContainerNode(&element.document())
     , m_element(&element)
     , m_name(name)
     , m_ignoreChildrenChanged(0)
 {
     ScriptWrappable::init(this);
 }
 
 Attr::Attr(Document& document, const QualifiedName& name, const AtomicString& standaloneValue)
     : ContainerNode(&document)
-    , m_element(0)
+    , m_element(nullptr)
     , m_name(name)
     , m_standaloneValue(standaloneValue)
     , m_ignoreChildrenChanged(0)
 {
     ScriptWrappable::init(this);
 }
 
 PassRefPtrWillBeRawPtr<Attr> Attr::create(Element& element, const QualifiedName& name)
 {
     RefPtrWillBeRawPtr<Attr> attr = adoptRefWillBeRefCountedGarbageCollected(new Attr(element, name));
@@ skipping 127 matching lines @@
     ASSERT(m_element);
     ASSERT(m_element->elementData());
     return *m_element->ensureUniqueElementData().getAttributeItem(qualifiedName());
 }
 
 void Attr::detachFromElementWithValue(const AtomicString& value)
 {
     ASSERT(m_element);
     ASSERT(m_standaloneValue.isNull());
     m_standaloneValue = value;
-    m_element = 0;
+    m_element = nullptr;
 }
 
 void Attr::attachToElement(Element* element)
 {
     ASSERT(!m_element);
     m_element = element;
     m_standaloneValue = nullAtom;
 }
 
+void Attr::trace(Visitor* visitor)
+{
+    visitor->trace(m_element);
+    ContainerNode::trace(visitor);
 }
+
+}