Service Worker: Tests for unregister then register

These are tests for the Chromium patch r288994:
https://codereview.chromium.org/413063004

Our current shortcomings are:
- New-scope registration overrides the existing registration. This is an
existing bug: https://code.google.com/p/chromium/issues/detail?id=398355
- register() should wait before resolving if docs are using an unregistered
registration. This is a new spec requirement.
- We don't do "early-rejection" described by
https://github.com/slightlyoff/ServiceWorker/issues/396. Also a new spec
requirement.

BUG=388095

Review URL: https://codereview.chromium.org/416003003

git-svn-id: svn://svn.chromium.org/blink/trunk@180141 bbb929c8-8fbe-4397-9dbb-9b2b20218538