Add 'chronos' to userlist

[h00die]

chronos is the default user for ChromeOS (I believe dev mode), but its not in the unix_users.txt.
Default password is 'password', which is already covered.

Reference: https://arnoldthebat.co.uk/wordpress/chromium-os/

I ordered a wrong replacement part for my laptop, so I won't be able to get to this for a week, but if someone new wants an easy one, here it is!!!!!