Handle "Next Track" in Picture-in-Picture window

This CL adds support for "Next Track" media session action in the
Picture-in-Picture window.

Screenshots: https://imgur.com/a/q8WPEAO

Bug: 926174
Change-Id: I4804cfa0b9218eb75d7bab6d5d9418eabf78cf2a
Reviewed-on: https://chromium-review.googlesource.com/c/1460917
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Reviewed-by: Eliot Courtney <edcourtney@chromium.org>
Commit-Queue: Fran├žois Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#631842}
18 files changed