Service Worker: add layout tests for behavior of controller after unregister

Unregister doesn't affect existing control over a document, but prevents the
worker from controlling on future document loads.

We already implement this behavior but no layout test existed yet.

BUG=388095

Review URL: https://codereview.chromium.org/358553003

git-svn-id: svn://svn.chromium.org/blink/trunk@177054 bbb929c8-8fbe-4397-9dbb-9b2b20218538