Adapt to removal of Mutter namespace

MutterWindow and MutterPlugin have been renamed to MetaWindowActor
and MetaPlugin, mutter_plugin_list_windows() to
meta_plugin_list_window_actors(). Adapt to those changes.

https://bugzilla.gnome.org/show_bug.cgi?id=632500

js/ui/chrome.js

@@ -233,7 +233,7 @@ Chrome.prototype = {
     },
 
     _windowsRestacked: function() {
-        let windows = global.get_windows();
+        let windows = global.get_window_actors();
         let primary = global.get_primary_monitor();
 
         // The chrome layer should be visible unless there is a window