I wonder in which language Chromium OS is written.I guess they have used C/C++ but did they put something different (Go)?
Did they used Assembly for low level code as I know that they had to change some things to make booting a lot faster?
Poke around /etc/ in Chromium, and you'll quickly see it is mostly Ubuntu; Google contracted with Canonical to do the majority of the work.
It boots quickly because it doesn't do much. :) I'm sure there's more to it than that, but restricting what the system can do is a great way to reduce the boot speed problem to something more tractable.