layout: make Chrome an implementation detail of LayoutManager

Make the Chrome object be a private member of LayoutManager, and add
new LayoutManager methods to proxy to the old Chrome methods.

https://bugzilla.gnome.org/show_bug.cgi?id=655813

js/ui/lookingGlass.js

@@ -752,7 +752,7 @@ LookingGlass.prototype = {
 
         // we add it to the chrome because we want it to appear to slide
         // out from underneath the panel
-        Main.chrome.addActor(this.actor);
+        Main.layoutManager.addChrome(this.actor);
 
         this._objInspector = new ObjInspector();
         Main.uiGroup.add_actor(this._objInspector.actor);