Code in chrome reaches into ash to call Shell::GetRootWindowForNewWindows(). With out-of-process ash it can't do that directly. But it can use Screen, which has a mojo impl to ash/window service.

I think display::GetDisplayIdForNewWindow() could be implemented this way:

ash::Shell (or an OnWindowActivated observer) ->
//services/ui/ws2/window_service.cc ->
ScreenProviderMus ->
(mojo)
ScreenMus

It could follow SetFrameDecorationValues. Maybe it should have its own mojo method to avoid sending the whole display list every time a window activates.