/usr/share/glib-2.0/schemas/10_libgtk-3-common.gschema.override is in libgtk-3-common 3.10.8-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | [org.gtk.Settings.FileChooser]
startup-mode='cwd'
|