GitLab

Adds flags for configuring the size of the rolling window and the number
of days to keep stats before expiring.

Bug: 910404
Test: new unit tests
Change-Id: I8a88a81154c710a1cc9b029e2b8ef8af61d945e0
Reviewed-on: https://chromium-review.googlesource.com/c/1355996Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612479}