Bug 1949881 [wpt PR 50885] - DOM: Implement finally() Observable operator, a=testonly

Automatic update from web-platform-tests
DOM: Implement `finally()` Observable operator

See WICG/observable#153. This CL implements the
`finally()` operator. It registers a callback which runs on both
producer-initiated and consumer-initiated unsubscription. Its
implementation is relatively straightforward, just utilizing the
relevant subscriber's teardown list.

This CL used to be really complicated, as was the spec PR. This has all
been simplified since the factors that made it complicated (and broken)
have been landed. See:
 - WICG/observable#151 (comment)
 - WICG/observable#154
 - https://crrev.com/c/5676226

Now that the simplifying factors have been landed, this CL can proceed
with a simple implementation, which passes all of the tests.

R=dizhanggchromium.org

Bug: 40282760
Change-Id: Ifff220ec094abd2187f35020eae85c1bca502f89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5654720
Reviewed-by: Di Zhang <dizhanggchromium.org>
Commit-Queue: Dominic Farolino <domchromium.org>
Cr-Commit-Position: refs/heads/main{#1423349}

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wpt-commits: 12acca9b67ce3bece2f1c19e1ea603d2d1d5a70d
wpt-pr: 50885

UltraBlame original commit: d312560aa22af3f36388be6c207b20066ad40525

Author: marco-c