GitLab

This metric will be used for a time to first byte metric in
Chrome User Experience Report.

UKM collection review: https://docs.google.com/document/d/1U0gT3RalytZqPW9t-4lBweBwLaamXYdJaO4ETsuvCdU/edit#heading=h.k5jx6iluw4yt

Change-Id: Ie311d28f98b90f4361845e10843fa50f066c32b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628048Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Bryan McQuade <bmcquade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663169}