Revert "loginDialog: Don't arbitrarily pick a random session for the user"

This reverts commit 6d40cb98e7.

https://bugzilla.gnome.org/show_bug.cgi?id=740142

js/gdm/loginDialog.js

@@ -354,6 +354,9 @@ const SessionMenuButton = new Lang.Class({
             this._menu.addMenuItem(item);
             this._items[id] = item;
 
+            if (!this._activeSessionId)
+                this.setActiveSession(id);
+
             item.connect('activate', Lang.bind(this, function() {
                 this.setActiveSession(id);
             }));