GitLab

Added explicit values to enum entries and added a comment mentioning to
not change or re-use those values.

Also made histogram definition clearer.

Change-Id: Idada23970bd834575d314bcfac941f5a3b0e255c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2480582
Auto-Submit: sebsg <sebsg@chromium.org>
Commit-Queue: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818019}