Announce commit or commits to test for internal and external runs

[foolip]

Background:

• The original Design Doc: WPT Dashboard had "one build per platform every morning that runs all WPT tests against one WPT revision" and then, as a simplification, "Instead of bots pulling the WPT revision to test against, they just test against the first WPT revision that day".
• Make test runner test against the first SHA of the day #40 is about that simplification although the original central "pick a revision" part no longer exists.
• Consistent results for all browsers are updated every 24 hours will require results from the same commit. "Picking a WPT commit" is part of the description, although it's not strictly required to achieve the objective.
• Test every commit of web-platform-tests within 1 hour #164 is about testing much more frequently than once a day.
• Support external runs (submitting results) for more browsers #290 is about supporting external runs.

Those are the "use cases" to solve. Proposed solution and title of this issue:

Have an endpoint like https://wpt.fyi/api/shas that returns at the very least a commit to test, but more usefully a set of commits which is either pre-prioritized or has enough information that consumers could decide which to prioritize running first.

The minimum useful behavior for runners would be to prioritize finishing a well-defined "today's sha".

[foolip]

@mattl, would you consider this resolved by https://sha.wpt.fyi/? I think a bit more would be required to support external runs, but it's fine to keep that to #290.

[jugglinmike]

As of gh-522, we collect results on a regular schedule (once every 6 hours for Chrome and Firefox, once every 2 days for Edge and Safari). We identify the latest commit to WPT at this moment, and all subsequent testing operations reference that commit via its git SHA sum.