Improve DEFINE_STATIC_LOCAL()'s handling of Blink GCed objects.

third_party/WebKit/Source/core/dom/TreeScope.cpp

 /*
  * Copyright (C) 2011 Google Inc. All Rights Reserved.
  * Copyright (C) 2012 Apple 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 152 matching lines @@
 {
     if (elementId.isEmpty())
         return 0;
     if (!m_elementsById)
         return 0;
     return m_elementsById->getElementById(elementId, this);
 }
 
 const HeapVector<Member<Element>>& TreeScope::getAllElementsById(const AtomicString& elementId) const
 {
-    DEFINE_STATIC_LOCAL(Persistent<HeapVector<Member<Element>>>, emptyVector, (new HeapVector<Member<Element>>()));
+    DEFINE_STATIC_LOCAL(HeapVector<Member<Element>>, emptyVector, (new HeapVector<Member<Element>>));
     if (elementId.isEmpty())
-        return *emptyVector;
+        return emptyVector;
     if (!m_elementsById)
-        return *emptyVector;
+        return emptyVector;
     return m_elementsById->getAllElementsById(elementId, this);
 }
 
 void TreeScope::addElementById(const AtomicString& elementId, Element* element)
 {
     if (!m_elementsById)
         m_elementsById = DocumentOrderedMap::create();
     m_elementsById->add(elementId, element);
     m_idTargetObserverRegistry->notifyObservers(elementId);
 }
@@ skipping 388 matching lines @@
     visitor->trace(m_idTargetObserverRegistry);
     visitor->trace(m_selection);
     visitor->trace(m_elementsById);
     visitor->trace(m_imageMapsByName);
     visitor->trace(m_labelsByForAttribute);
     visitor->trace(m_scopedStyleResolver);
     visitor->trace(m_radioButtonGroupScope);
 }
 
 } // namespace blink