I think this was needed when we had coordinated graphics and coordinated graphics threaded, but now we can just make LayerTreeHost inherit from ThreadedCompositor::Client and ThreadedDisplayRefreshMonitor::Client.
Pull request: https://github.com/WebKit/WebKit/pull/5071
Committed 255214@main (1dfb430ef238): <https://commits.webkit.org/255214@main> Reviewed commits have been landed. Closing PR #5071 and removing active labels.