ServiceWorker: Fix assertion failure in the dtor of RespondWithObserver

Pending state in the dtor is valid. This change removes the assertion and
sends null response instead.


BUG=394203
TEST=run_webkit_tests.py http/tests/serviceworker/fetch-event.html

Review URL: https://codereview.chromium.org/400943002

git-svn-id: svn://svn.chromium.org/blink/trunk@178623 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix Service Worker unregistration test to test what it intended to test

The "Unregister does not affect controller" test didn't actually
unregister the service worker.

Also conform to the new Service Worker test style guide:
https://sites.google.com/a/chromium.org/dev/blink/serviceworker/testing

BUG=388095

Review URL: https://codereview.chromium.org/404733003

git-svn-id: svn://svn.chromium.org/blink/trunk@178624 bbb929c8-8fbe-4397-9dbb-9b2b20218538