This allows us to remove some duplicated code in classes derived from ScrollingTreeScrollingNode.
Pull request: https://github.com/WebKit/WebKit/pull/5131
Committed 255323@main (743a5cdc9032): <https://commits.webkit.org/255323@main> Reviewed commits have been landed. Closing PR #5131 and removing active labels.
<rdar://problem/100945030>