GitLab

Add support for Previous/NextTrack to MediaController
and the underlying MediaSession. This will be mapped
to the previous and next track media keys on CrOS.

BUG=893296

Change-Id: I582cd70a04c24af69c80ca891a281311b4d05434
Reviewed-on: https://chromium-review.googlesource.com/c/1287268Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603528}