Make HTMLCollection / NodeList backward traversal consistent with forward one

Source/core/dom/NodeTraversal.cpp

Index: Source/core/dom/NodeTraversal.cpp
diff --git a/Source/core/dom/NodeTraversal.cpp b/Source/core/dom/NodeTraversal.cpp
index 37e37cc38814a9f3a022d2deefab6ff67a56f757..0f4a9f16a088c5522ada0984c9f3606287d37b00 100644
--- a/Source/core/dom/NodeTraversal.cpp
+++ b/Source/core/dom/NodeTraversal.cpp
@@ -96,6 +96,14 @@ Node* NodeTraversal::nextAncestorSibling(const Node& current, const Node* stayWi
     return 0;
 }
 
+Node* NodeTraversal::lastWithin(const ContainerNode& current)
+{
+    Node* descendant = current.lastChild();
+    for (Node* child = descendant; child; child = child->lastChild())
+        descendant = child;
+    return descendant;
+}
+
 Node* NodeTraversal::previous(const Node& current, const Node* stayWithin)
 {
     if (current == stayWithin)