[UI-side compositing] Have ScrollerPairMac::setScrollbarStyle update the NSScrollerImp style as well
<rdar://problem/107334257>
Pull request: https://github.com/WebKit/WebKit/pull/12072
Committed 262253@main (f0a92ce4e218): <https://commits.webkit.org/262253@main> Reviewed commits have been landed. Closing PR #12072 and removing active labels.