We should use WeakHashSet for transientMutationObserverRegistry instead of HashSet of raw pointers.
Pull request: https://github.com/WebKit/WebKit/pull/9536
Committed 259793@main (319370e5b8a1): <https://commits.webkit.org/259793@main> Reviewed commits have been landed. Closing PR #9536 and removing active labels.
<rdar://problem/104989075>