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

Third attempt.  Previous versions here:
https://chromium-review.googlesource.com/c/chromium/src/+/1407303
https://chromium-review.googlesource.com/c/chromium/src/+/1372666

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: Icc2f8d618b86ad2c422de3d11fdb1f870453968b
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-Original-Original-Commit-Position: refs/heads/master@{#622254}
Reviewed-on: https://chromium-review.googlesource.com/c/1407303
Cr-Original-Commit-Position: refs/heads/master@{#622597}
Reviewed-on: https://chromium-review.googlesource.com/c/1419204
Cr-Commit-Position: refs/heads/master@{#624402}
32 files changed