GitLab

Adds a mojo AudioFocus API that can be used to access
AudioFocusManager inside the media session service.

This also removes the RequestId when requesting or
abandoning audio focus and replaces it with a
AudioFocusRequestClient interface. We still keep the
ID around though as it is useful for testing and
identifying requests.

BUG=875004

Change-Id: I88374a08a14171f747a120beaa380c3b126bfb4f
Reviewed-on: https://chromium-review.googlesource.com/c/1199923
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: apacible <apacible@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596404}