Always unregister ServiceWorker after each layout test

This change seems important to make tests non-flaky

Before this change, running SW tests with --iterations=10:
* current-on-reload.html occasionally fails (9/10 times)
* postmessage-msgport-to-client.html occasionally times out (8/10 times)
* indexeddb.html occasionally times out (1/10 times)

After this change:
* indexeddb.html occasionally times out (5/10 times) ??

Increasing # of iterations doesn't change the failure rate much.
(Haven't investigated the indexeddb.html flakiness)

BUG=370777,
TEST=virtual/serviceworker/http/tests with --iterations=N

Review URL: https://codereview.chromium.org/283463002

git-svn-id: svn://svn.chromium.org/blink/trunk@173940 bbb929c8-8fbe-4397-9dbb-9b2b20218538