/usr/share/lightdm/lightdm.conf.d/40-lightdm-webkit-greeter.conf is in lightdm-webkit-greeter 0.1.2-0ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | [SeatDefaults]
greeter-session=lightdm-webkit-greeter
|