/etc/signon-ui/webkit-options.d/www.facebook.com.conf is in account-plugin-facebook 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 3 4 5 6 | ViewportWidth = 420
ViewportHeight = 320
UsernameField = input[name="email"]
PasswordField = input[name="pass"]
#ZoomFactor = 2
#PreferredWidth = 420
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