MediaCapabilities: Add unweighted averages to DB for experimenting

In-memory DB experimentation by liberato@ shows an unweighted
approach may improve smooth=false recall and precision. This adds new
fields / logic to the database for storage/retrieval of unweighted
averages so we can run these experiments on a larger population with
a persistent (on disk) database.

Bug: 967016
Change-Id: If39b0515a7c7ab3ec8997f2f27f8217a99710a7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628814
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663339}
5 files changed