Plumb synchronous scrolling reason changed through RemoteScrollingTree for ScrollPerf
<rdar://problem/102828256>
Pull request: https://github.com/WebKit/WebKit/pull/6986
Committed 257388@main (b7bf1e1b226b): <https://commits.webkit.org/257388@main> Reviewed commits have been landed. Closing PR #6986 and removing active labels.