Bug 1865925 [wpt PR 43265] - DOM: Introduce Subscriber#addTeardown() …

…for Observables, a=testonly

Automatic update from web-platform-tests
DOM: Introduce Subscriber#addTeardown() for Observables

See https://github.com/WICG/observable/blob/master/README.md and the
design discussion in WICG/observable#22.

For WPTs:
Co-authored-by: benbenlesh.com

R=masonfchromium.org

Bug: 1485981
Change-Id: I1de6ff5813f1c9ea6762fb4569440350c5dc38ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5018147
Commit-Queue: Dominic Farolino <domchromium.org>
Reviewed-by: Mason Freed <masonfchromium.org>
Cr-Commit-Position: refs/heads/main{#1227535}

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wpt-commits: 6bbd0ddcb078ca56533085359b4b5fa5ed36ee2f
wpt-pr: 43265

UltraBlame original commit: 5d1528c1841d314e74a855ee483fcca04ddf1cd8