/usr/share/lightdm/lightdm.conf.d/55-unity8.conf is in unity8-desktop-session-mir 1.0.12+15.10.20150609-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | [SeatDefaults]
user-session=unity8-mir
unity-compositor-command=unity-system-compositor.sleep
session-wrapper=lightdm-unity8-session
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