GitLab

Migrated two metrics to use new APIs and flagged other identified
histograms' descriptions to highlight issue on dashboards (I will
email relevant owners after this lands).
Relevant metrics list extracted from https://crbug.com/807615#c19

R=asvitkine@chromium.org, fdoray@chromium.org

Bug: 807615
Change-Id: I84f0f8fb5ca7fd3c5b43ba7209693b89928ade07
Reviewed-on: https://chromium-review.googlesource.com/1062830
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560704}