[Feature] add a webkit/firefox auto-roll bots

[aslushnikov]

The idea is to have a bot that tries to roll browsers nightly and runs tests against the new build.

• At the very least, it should report when things succeed and when they don't.
• Additionally, it might submit a PR with a new roll. We're free to merge the PR if we like so.

[aslushnikov]

These are deployed on https://devops.aslushnikov.com.