The views::CreateDesktopScreen() function in ui/views/widget/desktop_aura/desktop_screen.h creates a useful display::Screen for the desktop environment. It's implemented as DesktopScreenX11 and DesktopScreenWin:
- ui/views/widget/desktop_aura/desktop_screen_x11.h
- ui/views/widget/desktop_aura/desktop_screen_win.h

These actually don't make much use of views, and they're useful with vanilla aura::WindowTreeHosts, not just views subclasses like views::DesktopWindowTreeHostX11.

Since DesktopScreen will be used in AppShell, the non-views-specific parts of its implementations should be moved outside of //ui/views in order to
  a) reduce unnecessary dependencies on views, and
  b) ensure DesktopScreen implementations don't accidentally assume that an aura window is an instance of a views subclass.

From sadrul@ in https://chromium-review.googlesource.com/c/614460/6/extensions/shell/browser/shell_desktop_controller_aura.cc#262:

> My only concern is that the views implementation may assume things to be of
> certain types (e.g. always expect a WindowTreeHost to be a
> DesktopWindowTreeHost, etc.), even if not right now, in some unfortunate
> future change. So I would encourage you to split out the relevant parts of
> DesktopScreenX11/DesktopScreenWin out of views, and into //ui/display/[x11|win],
> so we don't have to worry about this. (please file a crbug and a TODO here
> referencing that)