Drop unnecessary NodeWithIndex::m_haveIndex data member

Source/core/dom/NodeWithIndex.h

Index: Source/core/dom/NodeWithIndex.h
diff --git a/Source/core/dom/NodeWithIndex.h b/Source/core/dom/NodeWithIndex.h
index 02b6d33c15acdadcd3abdffe27003c6f37e2500e..7efddfc8aff82e84c90d1e7dbbbbabccfa616f8a 100644
--- a/Source/core/dom/NodeWithIndex.h
+++ b/Source/core/dom/NodeWithIndex.h
@@ -36,7 +36,7 @@ class NodeWithIndex {
 public:
     explicit NodeWithIndex(Node& node)
         : m_node(node)
-        , m_haveIndex(false)
+        , m_index(-1)
     {
     }
 
@@ -44,17 +44,17 @@ public:
 
     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;
 };