[UI-side compositing] Add helper function to ui-helper.js to get scrollbar state
<rdar://problem/107332737>
Pull request: https://github.com/WebKit/WebKit/pull/12069
Committed 262246@main (9ca2fd55c226): <https://commits.webkit.org/262246@main> Reviewed commits have been landed. Closing PR #12069 and removing active labels.