[Ui-side compositing] Fix mouse position reporting to AppKit in WebScrollerImpPairDelegateMac
<rdar://problem/107776583>
Pull request: https://github.com/WebKit/WebKit/pull/12542
Committed 262995@main (caec1d312c59): <https://commits.webkit.org/262995@main> Reviewed commits have been landed. Closing PR #12542 and removing active labels.