main, LoginManager: Call GDM's RegisterSession()

So that it can know if we started up properly and use that to (e.g.)
kill its greeter.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/570

js/ui/main.js

@@ -250,6 +250,8 @@ function _initializeUI() {
             });
         }
 
+        LoginManager.registerSessionWithGDM();
+
         let perfModuleName = GLib.getenv("SHELL_PERF_MODULE");
         if (perfModuleName) {
             let perfOutput = GLib.getenv("SHELL_PERF_OUTPUT");