layout: Don't unnecessarily go through Main for a property

We *are* Main.layoutManager, here.

https://bugzilla.gnome.org/show_bug.cgi?id=682429

js/ui/layout.js

@@ -314,7 +314,7 @@ const LayoutManager = new Lang.Class({
 
     get currentMonitor() {
         let index = global.screen.get_current_monitor();
-        return Main.layoutManager.monitors[index];
+        return this.monitors[index];
     },
 
     _startupAnimation: function() {