Make HTMLCollection / NodeList backward traversal consistent with forward one

Source/core/dom/ParentNode.h

Index: Source/core/dom/ParentNode.h
diff --git a/Source/core/dom/ParentNode.h b/Source/core/dom/ParentNode.h
index dda6a8afb7da6170f1c740b3dee34ec320dee0ef..7d6a67f298b498fb0c51fd56233bb4bda23d05cd 100644
--- a/Source/core/dom/ParentNode.h
+++ b/Source/core/dom/ParentNode.h
@@ -45,12 +45,12 @@ public:
 
     static Element* firstElementChild(ContainerNode& node)
     {
-        return ElementTraversal::firstWithin(node);
+        return ElementTraversal::firstChild(node);
     }
 
     static Element* lastElementChild(ContainerNode& node)
     {
-        return ElementTraversal::lastWithin(node);
+        return ElementTraversal::lastChild(node);
     }
 
     static unsigned childElementCount(ContainerNode& node)