Drop unnecessary NodeWithIndex::m_haveIndex data member

Source/core/dom/NodeWithIndex.h

 /*
  * Copyright (C) 2008 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 18 matching lines @@
 #include "core/dom/Node.h"
 
 namespace WebCore {
 
 // For use when you want to get the index for a node repeatedly and
 // only want to walk the child list to figure out the index once.
 class NodeWithIndex {
 public:
     explicit NodeWithIndex(Node& node)
         : m_node(node)
-        , m_haveIndex(false)
+        , m_index(-1)
     {
     }
 
     Node& node() const { return m_node; }
 
     int index() const
     {
-        if (!m_haveIndex) {
+        if (!hasIndex())
             m_index = m_node.nodeIndex();
-            m_haveIndex = true;
-        }
+        ASSERT(hasIndex());
         ASSERT(m_index == static_cast<int>(m_node.nodeIndex()));
         return m_index;
     }
 
 private:
+    bool hasIndex() const { return m_index >= 0; }
+
     Node& m_node;
-    mutable bool m_haveIndex;
     mutable int m_index;
 };
 
 }
 
 #endif