ChromiumOS is what Google bases ChromeOS on, and it's source is available (most notably, the U-Boot and device-specific firmware source is available for all Chromebooks). That's one of the reasons why Chromebooks are so well-supported by coreboot.
"For years, coreboot has been struggling against Intel. Intel has been shown to be extremely uncooperative in general. Many coreboot developers, and companies, have tried to get Intel to cooperate; namely, releasing source code for the firmware components. Even Google, which sells millions of chromebooks (coreboot preinstalled) have been unable to persuade them.
...
Basically, all Intel hardware from year 2010 and beyond will never be supported by libreboot...."
