There is no stable sandboxed browser protecting against kernel vulnerabilities on any platform that I know of prior to Chrome. That's the very definition of pioneering. Do you have an example of prior art? There may be sandboxing mechanisms in various kernels, but that's not what browser users care about.

As for the other points:

* "seccomp-nonbpf" is a vastly more limited mechanism than seccomp-bfp, and inappropriate for Chrome use-case. http://en.wikipedia.org/wiki/Seccomp. seccomp-bfp is pioneering in Linux kernel space, even if, ironically, the team might not have wanted to spend their time there. See below.

* Popular consumer distribution of Linux doesn't have SELinux enabled by default. https://wiki.ubuntu.com/SELinux. "SELinux can be enabled in Ubuntu by installing the "selinux" meta-package". In my bubble we everyone uses Ubuntu on desktop, apologies to other distributions vying for the title of "popular consumer Linux distribution".

* Not sure why we are talking about security mechanisms on non-Linux kernels. Chrome is a browser, the team's job is not to innovate in the kernel space. http://www.chromium.org/developers/design-documents/sandbox. "Do not re-invent the wheel: It is tempting to extend the OS kernel with a better security model. Don't.".