/etc/signon-ui/webkit-options.d/api.twitter.com.conf is in account-plugin-twitter 0.12+16.04.20160126-0ubuntu1.
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_or_email"]
PasswordField = input[id="password"]
|