Add 'Mac' subclasses to RemoteLayerTreeDrawingArea{Proxy}, and RemoteScrollingCoordinatorProx, which will handle macOS-specific code for UI-side compositing.
<rdar://problem/101048413>
Pull request: https://github.com/WebKit/WebKit/pull/5247
Committed 255411@main (3abb691bf7d8): <https://commits.webkit.org/255411@main> Reviewed commits have been landed. Closing PR #5247 and removing active labels.