[Cast MRP] Connect CastMRP to CastActivityManager.

With this patch, it will now be possible to launch and terminate a
Cast app using the native CastMRP. Note that only the initial sequence
of Cast messages exchanged between the SDK/MRP/receiver for session
launch is supported, e.g. media commands, receiver status, etc., will
be implemented in a future patch.

Bug: 809249
Change-Id: Id9639e2523b91dbb0252c39b2e9bcae76a1fbb4a
Reviewed-on: https://chromium-review.googlesource.com/1168388
Commit-Queue: Derek Cheng <imcheng@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591885}
9 files changed