VideoTrackGenerator writer should get closed when its generator track (and all its clones) are stopped
<rdar://problem/121835553>
https://github.com/WebKit/WebKit/pull/23404
Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/44289
Committed 273778@main (8783423626ac): <https://commits.webkit.org/273778@main> Reviewed commits have been landed. Closing PR #23404 and removing active labels.