I didn't consider it, but I believe that using CPU for protection (ring 1/2)
would require changes in the kernel. The beauty of seccomp is that it's been
in the kernel for several years now and is quite widely deployed.
Also, you're correct that all network IO runs in the main browser process.
This is actually a little unfortunate: it would be best to have a separate,
sandboxed process for that but, alas, that's only a wishlist item for now.