GitLab

Add two UKM fields to Media.Engagement.SessionFinished to
track the total playbacks from audio contexts and media
elements.

BUG=887619

Change-Id: Icb1ed1300114b09737bab2393b5c3d40e870fc5f
Reviewed-on: https://chromium-review.googlesource.com/c/1236924Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597149}