Use WeakHashSet<ScrollableArea> instead of HashSet<ScrollableArea*>
<rdar://problem/103418459>
Pull request: https://github.com/WebKit/WebKit/pull/7719
Committed 258255@main (90f1d32abbce): <https://commits.webkit.org/258255@main> Reviewed commits have been landed. Closing PR #7719 and removing active labels.