/usr/share/lightdm/lightdm.conf.d/50-guest-wrapper.conf is in lightdm 1.18.3-0ubuntu1.1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | [Seat:*]
guest-wrapper=/usr/lib/lightdm/lightdm-guest-session
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