This patch implements the ready attribute on ServiceWorkerContainer,
a Promise that resolves when there's a ServiceWorkerRegistration
with an active worker for the current page. The promise never rejects
and stays pending if no active service worker exists.
This makes https://instagram.com/ load once again :^)
Now we can register jobs and they will be executed on the event loop
"later". This doesn't feel like the right place to execute them, but
the spec needs some updates in this regard anyway.