It's like WeakHashSet, but thread safe!
Pull request: https://github.com/WebKit/WebKit/pull/7377
Committed 257817@main (1d67c0b3b7db): <https://commits.webkit.org/257817@main> Reviewed commits have been landed. Closing PR #7377 and removing active labels.
<rdar://problem/103326713>
Reverted by https://github.com/WebKit/WebKit/pull/7591
Committed 257830@main (c08ef2f6eaf5): <https://commits.webkit.org/257830@main> Reviewed commits have been landed. Closing PR #7591 and removing active labels.
Redoing in https://bugs.webkit.org/show_bug.cgi?id=249350