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The CloseNewestToExitConcurrentPlayback histogram was not
being recorded because of a race condition between closing
the tab and becoming inaudible and the web contents being
destroyed. This changes the metrics to check if a web
contents was recently audible instead of whether it is
currently audible.

BUG=936196

Change-Id: Ie3c1d8bbb0e11b099acbc2c163aeedf4b1c9d58f
Reviewed-on: https://chromium-review.googlesource.com/c/1490494Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636274}