Comment 10 for bug 1523929

That command should be run automatically when you installed libgtk-3-dev (or any other package containing schemas). So I guess the real question here is why it did not trigger it when you installed. I am curious if it is reproducible can you try removing libgtk-3-dev (check the schema key has gone away), then reinstall libgtk-3-dev and check if the key is back (without that command)