layout: Mark chrome container as NO_LAYOUT

Showing and hiding children does not affect the allocation of the uiGroup
nor its other children. We can avoid full relayout/redraw in those
situations.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/197


(cherry picked from commit 73d8c82640)

js/ui/layout.js

@@ -203,6 +203,7 @@ var LayoutManager = new Lang.Class({
 
         // Set up stage hierarchy to group all UI actors under one container.
         this.uiGroup = new Shell.GenericContainer({ name: 'uiGroup' });
+        this.uiGroup.set_flags(Clutter.ActorFlags.NO_LAYOUT);
         this.uiGroup.connect('allocate', (actor, box, flags) => {
             let children = actor.get_children();
             for (let i = 0; i < children.length; i++)