/etc/signon-ui/webkit-options.d/foursquare.com.conf is in account-plugin-foursquare 0.11+14.04.20140409.1-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | UsernameField = input[id="username"]
PasswordField = input[id="password"]
|