GitLab

Updates the GPU documentation on adding new builders/testers to clarify
what to do if adding both a new parent builder and a tester triggered by
it, as it turns out that it's not as simple as performing the
tester-only steps two separate times.

Change-Id: Iddb3336db0fc12a22e8cb73ac6aa3dffe13f0ffe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2613667
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840824}