Remove children from the end to prevent copying over

Summary:
This PR leads to removing the children from the end. [So we don't have the overhead of copying them to the front](dcaef7ecb0/CSSLayout/CSSNodeList.c (L62)).
Closes https://github.com/facebook/css-layout/pull/233

Reviewed By: emilsjolander

Differential Revision: D4075905

Pulled By: splhack

fbshipit-source-id: d8b460badb01bfc6ea647004c799395b9cab9e7c

csharp/Facebook.CSSLayout/CSSNode.cs

@@ -486,7 +486,7 @@ namespace Facebook.CSSLayout
         {
             while (_children.Count > 0)
             {
-                RemoveAt(0);
+                RemoveAt(_children.Count-1);
             }
         }