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Add UMA metrics for tracking audio focus events. There are three
histograms added in total; the first records RequestAudioFocus
calls, the second records AbandonAudioFocus calls and the third
records requested AudioFocusTypes.

In the future we will probably switch these with UKM but UKM is
not available for ARC++ yet.

BUG=883958

Change-Id: I1092b9d1c638a13526687ac77b87d66490e4b89c
Reviewed-on: https://chromium-review.googlesource.com/c/1238871
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597337}