Remove redundant m_isItemCacheValid flag from LiveNodeListBase

Source/core/dom/LiveNodeList.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2006, 2007, 2008, 2010 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
  * version 2 of the License, or (at your option) any later version.
@@ skipping 29 matching lines @@
     Node& rootNode = this->rootNode();
     if (!rootNode.isContainerNode())
         return 0;
     return toContainerNode(&rootNode);
 }
 
 void LiveNodeListBase::invalidateCache() const
 {
     m_cachedItem = 0;
     m_isLengthCacheValid = false;
-    m_isItemCacheValid = false;
 }
 
 void LiveNodeListBase::invalidateIdNameCacheMaps() const
 {
     ASSERT(hasIdNameCache());
     static_cast<const HTMLCollection*>(this)->invalidateIdNameCacheMaps();
 }
 
 Node* LiveNodeList::namedItem(const AtomicString& elementId) const
 {
@@ skipping 13 matching lines @@
         Node* node = item(i);
         // FIXME: This should probably be using getIdAttribute instead of idForStyleResolution.
         if (node->hasID() && toElement(node)->idForStyleResolution() == elementId)
             return node;
     }
 
     return 0;
 }
 
 } // namespace WebCore