GitLab

Also a more useful error message if incorrect.
This makes it works as documented in docs/origin_trials_integration.md

Change-Id: I97d549d10917911d636fc8d41ebd58b1f32838de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415750
Auto-Submit: Glen Robertson <glenrob@chromium.org>
Commit-Queue: Jason Chase <chasej@chromium.org>
Reviewed-by: Jason Chase <chasej@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807898}