/usr/share/gnome-session/sessions/gnome-flashback-metacity.session is in gnome-session-flashback 1:3.18.2-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | [GNOME Session]
Name=GNOME Flashback (Metacity)
RequiredComponents=gnome-flashback-init;gnome-flashback;gnome-panel;metacity;unity-settings-daemon;gnome-flashback-services;
|