ChromeOS is open-source. Last I looked, their build process was a horror beyond all reason. However, it would at least be possible to strip out any advertising built into the OS. Thing is though, Google controls the advertising on the top two pages on the Internet, and as long as that's true, they don't really need to do anything other than make sure you get there. Which is the strategy behind Google Fiber, Loon, and to some degree Android, more-or-less the reason for Chrome's existence, and why Chromebooks are designed around constant Internet access: all roads lead to Rome.