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This CL implements the audibility timer for multiple AudioContexts
in AudioContextManagerImpl. The "started" event triggers the
tracking and the "stopped" event triggers the calculation of
elapsed time followed by UKM data record.

Bug: 896944
Change-Id: Id1f635df4783247aca3656d07c02af907ffd72e4
Reviewed-on: https://chromium-review.googlesource.com/c/1289695
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601446}