[whatwg/fetch] Deferred fetching (PR #1647)

Add a JS-exposed function to request a deferred fetch.

A deferred fetch would be invoked in one of two scenarios:
- The document is destroyed (the fetch group is terminated)
- The document is backgrounded (the fetch group is deactivated) and not restored after a certain time.

A few constraints:
- Deferred fetch body sizes are limited to 64KB per origin. Exceeding this would immediately reject with a QuotaExceeded.
- Request body streams are not allowed. A request body, if exists, has to be a byte sequence.

The JS function is called `requestDeferredFetch` but that's bikesheddable.

See https://github.com/WICG/pending-beacon/issues/70

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(See [WHATWG Working Mode: Changes](https://whatwg.org/working-mode#changes) for more details.)

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

  https://github.com/whatwg/fetch/pull/1647

-- Commit Summary --

  * Deferred fetching

-- File Changes --

    M fetch.bs (215)

-- Patch Links --

https://github.com/whatwg/fetch/pull/1647.patch
https://github.com/whatwg/fetch/pull/1647.diff

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