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Metrics show that the DB makes its best predictions for a given key
after accumulating ~2500 frames. With this change rows in the DB will
begin to down-weight the existing frames after reaching the 2500 frame
max. New appends of more than 2500 frames will completely flush out old
stats. This roughly approximates a sliding window of the last 2500
frames without adding complexity to the DB of a real sliding window.

This change also expires old statistics that have not received an
update in the last 30 days. This ensures that clients who have a bad
outlier decode performance aren't punished forever - otherwise sites
may never give them another chance at that resolution / frame-rate.

Change-Id: I82f6920fbe3f594dad36d35b12c55d32e65de5a6
Bug: 818889
Test: New unit tests
Reviewed-on: https://chromium-review.googlesource.com/1113503
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594427}