overview: Fix ShellInfo.setMessage()

Commit 22eea750 made info messages show up in the lock screen, but
as setShowWhenLocked() throws an exception when called on non-transient
notifications, the transient hint has to be set first.

https://bugzilla.gnome.org/show_bug.cgi?id=682268

js/ui/overview.js

@@ -70,14 +70,13 @@ const ShellInfo = new Lang.Class({
         let notification = null;
         if (this._source.notifications.length == 0) {
             notification = new MessageTray.Notification(this._source, text, null);
+            notification.setTransient(true);
             notification.setShowWhenLocked(true);
         } else {
             notification = this._source.notifications[0];
             notification.update(text, null, { clear: true });
         }
 
-        notification.setTransient(true);
-
         this._undoCallback = undoCallback;
         if (undoCallback) {
             notification.addButton('system-undo',