/usr/share/xsessions/gnome-flashback-metacity.desktop 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 4 5 6 7 8 | [Desktop Entry]
Name=GNOME Flashback (Metacity)
Comment=This session logs you into GNOME Flashback with Metacity
Exec=/usr/lib/gnome-flashback/gnome-flashback-metacity
TryExec=metacity
Type=Application
DesktopNames=GNOME-Flashback;Unity;
X-Ubuntu-Gettext-Domain=gnome-flashback
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