ServiceWorker: Make '.ready' return a promise to be resolved with ServiceWorkerRegistration (1/3)

This change makes navigator.serviceWorker.ready return a promise to be resolved
with ServiceWorkerRegistration object instead of ServiceWorker object.

Tests for '.ready' (ie. 'ready.html') are now disabled and those will be
re-enabled by the 3rd patch.

1) Blink: THIS PATCH
2) Chromium: https://codereview.chromium.org/477593007/
3) Blink: https://codereview.chromium.org/532653002/

Spec: https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-container


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

Review URL: https://codereview.chromium.org/476043002

git-svn-id: svn://svn.chromium.org/blink/trunk@181435 bbb929c8-8fbe-4397-9dbb-9b2b20218538