ServiceWorker: Enable ServiceWorkerRegistration and update layout tests

This enables ServiceWorkerRegistration interface and updates layout tests
to work with it.

NOTE: ServiceWorker feature is still behind a flag, so this doesn't break
any real web applications.

[Spec] https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-obj


BUG=396400
TEST=run_webkit_tests.py --debug http/tests/serviceworker

Review URL: https://codereview.chromium.org/466723002

git-svn-id: svn://svn.chromium.org/blink/trunk@180317 bbb929c8-8fbe-4397-9dbb-9b2b20218538