This adds presubmit checks and documentation for the features added on
infra side in https://crrev.com/c/1019080.

NOTRY=true

Bug: chromium:830557
Change-Id: If3808f5931f7b1043a8a80c03a293df4602ce758
Reviewed-on: https://chromium-review.googlesource.com/1021140Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52833}