[Cast MRP] STOP_SESSION message from client.

This patch is a continuation on patch 1419204 to implement STOP_SESSION
message.

Bug: 809249
Change-Id: I86b897a793e17b7b3495f8103904af3697f224ae
Reviewed-on: https://chromium-review.googlesource.com/c/1423425
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@{#634471}
7 files changed