- This implements an optimization that puts all children on
  non-dependent signals (i.e., those associated with a controller).
  This allows "intermediate" nodes (e.g., B in A follows B follows C)
  to be garbage collected if they are being kept alive to propagate
  aborts.

- This removes the follow algorithm, so callsites will need to be
  updated.

- The "create a composite abort signal" algorithm takes an interface so
  that TaskSignal.any() can easily hook into it, but create a 
  TaskSignal.

- Some algorithms that invoke "follow" create an AbortSignal in a 
  particular realm. This enables doing that, but borrows some language 
  from elsewhere in the spec w.r.t. doing the default thing. Neither of
  the other two static members specify a realm.

Follow-up PRs:

- whatwg/fetch#1646
- w3c/ServiceWorker#1678
- whatwg/streams#1277

This also sets the stage to make AbortSignal's "signal abort" fully 
internal. #1194 tracks the remainder.

Tests: web-platform-tests/wpt#37434 and web-platform-tests/wpt#39785.

Fixes #920.

Needed for whatwg/dom#1194.

Instead, create an AbortController per NavigateEvent and initialize the
event's signal to the controller's signal, using the controller to abort
the signal.

See also whatwg/dom#1194 and
WICG/navigation-api#265.

Bug: 1448352
Change-Id: I87d020ca7aab7396a7b7f2896bf886c4dd9e61b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/ /4562529
Commit-Queue: Scott Haseley <[email protected]>
Reviewed-by: Nate Chapin <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1161349}

Directly aborting signals violates assumptions for composite signals,
and should be reserved for AbortController and AbortSignal. This CL
guards SignalAbort with a pass key.

See also whatwg/dom#1194.

Bug: 1448352
Low-Coverage-Reason: Follow() is covered by virtual tests
Change-Id: I02cec638d1c4c20c6d85830ea5eb26b5cf4fd251
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/ /4638358
Commit-Queue: Scott Haseley <[email protected]>
Reviewed-by: Joey Arhar <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1161467}

Helps with #1194.