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Add grouped audio focus. This means that a media session
can provide an unguessable token when requesting audio
focus. This means that the media session service will
treat all media sessions with the same group id as the
same session when it comes to audio focus.

BUG=906285

Change-Id: I184f8de58cfe3cec67db0bb5877f3b352c794f13
Reviewed-on: https://chromium-review.googlesource.com/c/1342939
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611805}