userMenu: Update user name on session mode changes

The new privacy panel offers separate settings for the user name
visibility in lock screen and normal mode. The code to support
either setting is already in place, but we need to make sure to
update the user name on session mode changes to pick up the correct
one.

https://bugzilla.gnome.org/show_bug.cgi?id=690049

js/ui/userMenu.js

@@ -593,6 +593,7 @@ const UserMenuButton = new Lang.Class({
 
         this.setSensitive(!Main.sessionMode.isLocked);
         this._updatePresenceIcon();
+        this._updateUserName();
     },
 
     _onDestroy: function() {