Fix mistaken use of promise in multiple-register.html

service_worker_unregister_and_done was outside of the .then chain, so it
would execute early and the test passes regardless of the assertions.

BUG=395928

Review URL: https://codereview.chromium.org/426563003

git-svn-id: svn://svn.chromium.org/blink/trunk@179020 bbb929c8-8fbe-4397-9dbb-9b2b20218538