ServiceWorker: Stabilize install-phase-event-waituntil.html

'install-phase-event-waituntil.html' often causes a timeout on windows and
I suspect that the intended 'statechange' event could be fired before setting
the event handler.

To avoid timeouts, this CL makes the test use more stabilized wait_for_update()
helper introduced by
https://src.chromium.org/viewvc/blink?revision=182220&view=revision

BUG=405847
TEST=run_webkit_tests.py --debug http/tests/serviceworker/install-phase-event-waituntil.html

Review URL: https://codereview.chromium.org/580033002

git-svn-id: svn://svn.chromium.org/blink/trunk@182229 bbb929c8-8fbe-4397-9dbb-9b2b20218538