[UI-side compositing] Have RemoteScrollbarsController keep track of scrollbar visibility for hit testing
<rdar://problem/107859102>
Pull request: https://github.com/WebKit/WebKit/pull/12625
Committed 263004@main (a9d8f8b89517): <https://commits.webkit.org/263004@main> Reviewed commits have been landed. Closing PR #12625 and removing active labels.