[Cast MRP] Add CastActivityManager

CastActivityManager manages the lifetime of routes and corresponding
Cast sessions. This patch implements the minimum amount of message
handling necessary to launch a Cast session via CreateRoute() (to be
hooked up to CastMediaRouteProvider in next patch).

Bug: 809249
Change-Id: Ib6058a01d62113f8de1748cd2d9ca3dadabe8e49
Reviewed-on: https://chromium-review.googlesource.com/1159357
Commit-Queue: Derek Cheng <imcheng@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590027}
16 files changed