ServiceWorker: Clean up fetch-event.html

This change...
- removes unnecessary 'wait_for_update' calls because a register promise
is resolved with a registration object that has an installing worker.
- removes 't.step_func' from promise chains.
- unloads iframes gracefully.


BUG=401381
TEST=run_webkit_tests.py --debug http/tests/serviceworker/fetch-event.html

Review URL: https://codereview.chromium.org/531423002

git-svn-id: svn://svn.chromium.org/blink/trunk@181465 bbb929c8-8fbe-4397-9dbb-9b2b20218538