/usr/share/lightdm/lightdm.conf.d/55-unity8.conf is in unity8-desktop-session-mir 1.0.10+14.04.20140417.1-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | [SeatDefaults]
session-wrapper=lightdm-unity8-session
|