Make HTMLCollection / NodeList backward traversal consistent with forward one

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 67c61e488e368460f42fbb1a1ad298a2dec87864..bdc6e64e650328080aa1907ba2df083bc873f23a 100644
--- a/Source/core/dom/LiveNodeList.h
+++ b/Source/core/dom/LiveNodeList.h
@@ -50,9 +50,10 @@ public:
 
     // Collection IndexCache API.
     bool canTraverseBackward() const { return true; }
-    Element* itemBefore(const Element* previousItem) const;
     Element* traverseToFirstElement() const;
+    Element* traverseToLastElement() const;
     Element* traverseForwardToOffset(unsigned offset, Element& currentNode, unsigned& currentOffset) const;
+    Element* traverseBackwardToOffset(unsigned offset, Element& currentNode, unsigned& currentOffset) const;
 
 private:
     virtual Node* virtualOwnerNode() const OVERRIDE FINAL;