Promisify Service Worker test unregister_and_register helper function

Service Worker API is promise-based, so this helper should be too.

Also take care of a FIXME now that testharness has been updated.

BUG=
R=horo@chromium.org

Review URL: https://codereview.chromium.org/354953002

git-svn-id: svn://svn.chromium.org/blink/trunk@177030 bbb929c8-8fbe-4397-9dbb-9b2b20218538