[UI-Side compositing] Move ScrollerPairMac to ScrollingTreeScrollingNodeDelegateMac
<rdar://problem/104762822>
Pull request: https://github.com/WebKit/WebKit/pull/9258
Committed 259592@main (20982a6f9b4d): <https://commits.webkit.org/259592@main> Reviewed commits have been landed. Closing PR #9258 and removing active labels.