Make mac and nicosia delegates inherit from ThreadedScrollingTreeScrollingNodeDelegate and move the common code to the base class.
Pull request: https://github.com/WebKit/WebKit/pull/5132
Committed 255379@main (fb5313470ceb): <https://commits.webkit.org/255379@main> Reviewed commits have been landed. Closing PR #5132 and removing active labels.
<rdar://problem/101019795>