Expand to include code review for spec changes

[foolip]

Context: https://groups.google.com/a/chromium.org/d/msg/blink-dev/rspPrQHfFkI/Qfv4OvzBCQAJ

[foolip]

The implementation of this on the W3C side would be w3c/validate-repos#27.

[foolip]

Beyond enforcing the branch protection, I'm a bit of a pessimist and think a tool to audit changes in repos would also be needed, because lots of editors will have admin access to their repos and can push to master directly without really noticing that they're overriding branch protection.