[Cast MRP] Media commands, media status, and SET_VOLUME.

Forked from https://chromium-review.googlesource.com/c/chromium/src/+/1234371

This patch implements media commands sent from the SDK to the receiver,
as well as media status messages from the receiver to the SDK.

This patch also implements the SET_VOLUME message which sets the system
level volume and not considered part of media commands.

Bug: 809249
Change-Id: Ib6d0a8764ca16ec9f0c5748df16e2a7c3f972be0
Reviewed-on: https://chromium-review.googlesource.com/c/1372666
Commit-Queue: John Williams <jrw@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622254}
32 files changed