Remove unregister() from ServiceWorkerContainer

BUG=404064

Review URL: https://codereview.chromium.org/581293002

git-svn-id: svn://svn.chromium.org/blink/trunk@182300 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Use getRegistration in service_worker_unregister_and_register

Now it can be used with a worker that doesn't install successfully.

BUG=404064

Review URL: https://codereview.chromium.org/587573002

git-svn-id: svn://svn.chromium.org/blink/trunk@182301 bbb929c8-8fbe-4397-9dbb-9b2b20218538