[UI-side compositing] Fix for assert in scrollbar tests: scrollerImp == scroller->scrollerImp()
<rdar://problem/107380404>
Pull request: https://github.com/WebKit/WebKit/pull/12127
Committed 262301@main (3fb91e2c59ba): <https://commits.webkit.org/262301@main> Reviewed commits have been landed. Closing PR #12127 and removing active labels.