GitLab

Move AudioFocusObserver to the media session service and
convert it to use mojo. At the moment the observers
register themselves with AudioFocusManager but in the
future this will be with the Media Sesssion service.

Also adds an empty MediaSession mojo type so we can pass
it as an arg to AudioFocusObserver.

BUG=875004

Change-Id: I4bd09796018833cb9f9714b2296642bd40be48ff
Reviewed-on: https://chromium-review.googlesource.com/1180484
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Nick Carter <nick@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586012}