ChromeOS is not a web based OS in the sense of being a 'web app' and neither is it's display or windowing system. I don't think anyone here is using the same definition of 'web app'.
It is a Web app jungler OS without any trace of X.
In any case it was just an example, there are plenty of others where X existence is meaningless, replaced by Web apps, HTML 5 variant of Citrix, IoT interfaces, or the whole set of SaaS applications without any native presence on FOSS UNIX clones.
ChromeOS is a Web based OS with more market share than GNU/Linux, thus just by measuring its usage across US school system, it is higher than X.
Then we can add all remaining desktop and mobile OSes into the mix.
Finally, SaaS Web applications are in many cases the only way some applications ever reach FOSS UNIX clones.