Use `window.confirm` instead of `confirm`

Summary:
This silences the following JSHint warning:

```
>>> Lint for webroot/rsrc/js/application/uiexample/notification-example.js:

   Warning  (W117) JSHintW117
    'confirm' is not defined.

              39           notification.listen(
              40             'activate',
              41             function() {
    >>>       42               if (!confirm('Close notification?')) {
              43                 JX.Stratcom.context().kill();
              44               }
              45             });

```

Test Plan: `arc lint`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11065