main: register with session manager explicitly

Mutter now makes session registration an explicit required
step.  This is so we can tell the session manager when
we're ready to move on to the next phase.

This commit calls the new Meta.register_with_session() api after we're
initialized.

https://bugzilla.gnome.org/show_bug.cgi?id=694876

js/ui/main.js

@@ -168,6 +168,9 @@ function startSession() {
     // initiate logouts.
     EndSessionDialog.init();
 
+    // We're ready for the session manager to move to the next phase
+    Meta.register_with_session();
+
     _startDate = new Date();
 
     log('GNOME Shell started at ' + _startDate);