ServiceWorker: Make '.ready' return a promise to be resolved with ServiceWorkerRegistration (3/3)

This enables and tweaks ready.html to follow up the implementation changes.

1) Blink: https://codereview.chromium.org/476043002/
2) Chromium: https://codereview.chromium.org/477593007/
3) Blink: THIS PATCH


BUG=399533
TEST=run_webkit_tests.py --debug http/tests/serviceworker/

Review URL: https://codereview.chromium.org/532653002

git-svn-id: svn://svn.chromium.org/blink/trunk@181806 bbb929c8-8fbe-4397-9dbb-9b2b20218538