Auto merge of #7407 - nox:childnodes-replace, r=jdm

Optimise most basic case of .replaceChild() when updating childNodes



<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/7407)
<!-- Reviewable:end -->

components/script/dom/nodelist.rs

@@ -215,7 +215,10 @@ impl ChildrenList {
                     },
                 };
                 list.last_visited.set(Some(JS::from_ref(visited)));
-            } else {
+            } else if added.len() != 1 {
+                // The replaced child isn't the last visited one, and there are
+                // 0 or more than 1 nodes to replace it. Special care must be
+                // given to update the state of that ChildrenList.
                 match (prev, next) {
                     (Some(_), None) => {},
                     (None, Some(next)) => {