/usr/share/lightdm/lightdm.conf.d/10-unity-system-compositor.conf is in ubuntu-desktop-mir 0.4.3+16.04.20160323-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | [Seat:*]
type=unity;xlocal
unity-compositor-command=unity-system-compositor.sleep
|