This ports:
https://chromium-review.googlesource.com/c/493466/

The code was originally copied from bot_update. So were the bugs.

Bug: chromium:717504
NOTRY=true
TBR=tandrii@chromium.org,agable@chromium.org

Change-Id: If2d2dafdca8cd44f325dc770dfc42c17889a3b4a
Reviewed-on: https://chromium-review.googlesource.com/501787Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45224}