[whatwg/dom] Mark dependent abort signals as aborted before firing events (PR #1295)

The assert in 4.2.1 of "create a dependent abort signal" fails when creating a dependent signal while dispatching abort events or running abort algorithms if abort had not yet been propagated to one of the sources.

This fix splits "signal abort" into two phases: first, set the abort reason on the signal being aborted and all of its unaborted dependents; next, run the abort algorithms and dispatch events for the signal and those same dependents. Note that:
 1. Dependent signals do not themselves have dependent signals, which means it's unnecessary to recursively call "signal abort"
 2. This approach retains the existing event dispatch order, while ensuring the abort state is synced before any JS runs

This fixes #1293.

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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/dom/pull/1295

-- Commit Summary --

  * Mark dependent abort signals as aborted before firing events

-- File Changes --

    M dom.bs (34)

-- Patch Links --

https://github.com/whatwg/dom/pull/1295.patch
https://github.com/whatwg/dom/pull/1295.diff

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