The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/965ef5d47e085feb665f4761c99bcdba7e95706c

commit 965ef5d47e085feb665f4761c99bcdba7e95706c
Author: Becca Hughes <beccahughes@chromium.org>
Date: Thu Oct 04 21:22:13 2018

[Audio Focus] Add support for gain transient

Adds support for gain transient which will pause all
other media sessions when it gains focus. When it
loses focus the top most session will resume playing.

This is used for short media that should not play
over the top of other media sessions.

Also adds the Resume method to the MediaSession mojo
interface which is needed for resuming sessions.

BUG=891534

Change-Id: I4d00f2602c46c03680305f8f750294776afdbc24
Reviewed-on: https://chromium-review.googlesource.com/c/1258975
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596853}
[modify] https://crrev.com/965ef5d47e085feb665f4761c99bcdba7e95706c/content/browser/media/session/media_session_impl.cc
[modify] https://crrev.com/965ef5d47e085feb665f4761c99bcdba7e95706c/content/public/browser/media_session.h
[modify] https://crrev.com/965ef5d47e085feb665f4761c99bcdba7e95706c/services/media_session/audio_focus_manager.cc
[modify] https://crrev.com/965ef5d47e085feb665f4761c99bcdba7e95706c/services/media_session/audio_focus_manager.h
[modify] https://crrev.com/965ef5d47e085feb665f4761c99bcdba7e95706c/services/media_session/audio_focus_manager_unittest.cc
[modify] https://crrev.com/965ef5d47e085feb665f4761c99bcdba7e95706c/services/media_session/public/mojom/audio_focus.mojom
[modify] https://crrev.com/965ef5d47e085feb665f4761c99bcdba7e95706c/services/media_session/public/mojom/media_session.mojom