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Adds AudioContext player tracking to MediaEngagementContentsObserver
with it's own page level timer. On MediaEngagementSession this CL
seperates significant playback into media element playback and
audio context playback. Since we may record a playback we need to
disable committing on significant playback and just record on
destroy or navigate instead.

BUG=878460

Change-Id: Ic66152f8cb5b3a6338804c04de7dd0bf5c1cb154
Reviewed-on: https://chromium-review.googlesource.com/1194992
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587899}