[ServiceWorker] Add test for the hanging bug in loading ServiceWorker script.

Chromium side fix patch: https://codereview.chromium.org/640923003

BUG=419999

Review URL: https://codereview.chromium.org/650193003

git-svn-id: svn://svn.chromium.org/blink/trunk@183971 bbb929c8-8fbe-4397-9dbb-9b2b20218538