ServiceWorker: Add stress test for registration

This test makes sure that many service workers can work simultaneously without
any crashes.


BUG=392735
TEST=run_webkit_tests.py --details --debug http/tests/serviceworker/

Review URL: https://codereview.chromium.org/394133002

git-svn-id: svn://svn.chromium.org/blink/trunk@178359 bbb929c8-8fbe-4397-9dbb-9b2b20218538