Have HTMLCollection::item() return an Element as per specification

Source/core/dom/LiveNodeList.h

Index: Source/core/dom/LiveNodeList.h
diff --git a/Source/core/dom/LiveNodeList.h b/Source/core/dom/LiveNodeList.h
index e7026bf381edd4d6e5496023b1c191e2b7c3b121..45b96fc52e008cc3a4039188b554f895d5ccf1bf 100644
--- a/Source/core/dom/LiveNodeList.h
+++ b/Source/core/dom/LiveNodeList.h
@@ -76,7 +76,6 @@ public:
 
     ContainerNode& rootNode() const;
     bool overridesItemAfter() const { return m_overridesItemAfter; }
-    Node* itemBefore(const Node* previousItem) const;
 
     ALWAYS_INLINE bool hasIdNameCache() const { return !isLiveNodeListType(type()); }
     ALWAYS_INLINE bool isRootedAtDocument() const { return m_rootType == NodeListIsRootedAtDocument || m_rootType == NodeListIsRootedAtDocumentIfOwnerHasItemrefAttr; }
@@ -100,8 +99,12 @@ protected:
     ALWAYS_INLINE NodeListRootType rootType() const { return static_cast<NodeListRootType>(m_rootType); }
     bool shouldOnlyIncludeDirectChildren() const { return m_shouldOnlyIncludeDirectChildren; }
 
+    template <typename Collection>
+    static Element* iterateForPreviousNode(const Collection&, Node* current);
+    template <typename Collection>
+    static Element* itemBefore(const Collection&, const Node* previousItem);
+
 private:
-    Node* iterateForPreviousNode(Node* current) const;
     void invalidateIdNameCacheMaps() const;
 
     RefPtr<ContainerNode> m_ownerNode; // Cannot be null.
@@ -156,13 +159,14 @@ public:
     virtual void invalidateCache() const OVERRIDE FINAL;
 
     // Collection IndexCache API.
+    Node* itemBefore(const Node* previousItem) const;
     Node* traverseToFirstElement(const ContainerNode& root) const;
     Node* traverseForwardToOffset(unsigned offset, Node& currentNode, unsigned& currentOffset, const ContainerNode& root) const;
 
 private:
     virtual bool isLiveNodeList() const OVERRIDE FINAL { return true; }
 
-    mutable CollectionIndexCache<LiveNodeList> m_collectionIndexCache;
+    mutable CollectionIndexCache<LiveNodeList, Node> m_collectionIndexCache;
 };
 
 } // namespace WebCore