/etc/signon-ui/webkit-options.d/localhost.conf is in uoa-integration-tests 0.2+15.04.20150413.3-0ubuntu1.
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="username"]
PasswordField = input[name="password"]
#ZoomFactor = 2
#PreferredWidth = 420
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