[w3c/ServiceWorker] Make serviceWorker.ready spec match implementations (#1478)

This changes the spec to delay evaluating `ready promise` until `serviceWorker.ready` has been accessed at least once. Once `serviceWorker.ready` has been accessed, it always returns the same promise (just like we always did).

Fixes #1477
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/ServiceWorker/pull/1478

-- Commit Summary --

  * Make serviceWorker.ready spec match implementations

-- File Changes --

    M docs/index.bs (5)

-- Patch Links --

https://github.com/w3c/ServiceWorker/pull/1478.patch
https://github.com/w3c/ServiceWorker/pull/1478.diff

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