[Cast MRP] In-browser Cast MRP part 1.

This patch introduces message formats for launching sessions on a Cast
device and corresponding CastMessageHandler API. It also adds APIs for
opening a virtual connection for a (sourceId,destId) pair, and
forwarding an app message from Cast SDK client to the device.

Also organized cast_message_utils to map message types to strings.

Bug: 809249
Change-Id: I63d4f658e53579d6a761a8857508534ad993ebc4
Reviewed-on: https://chromium-review.googlesource.com/1060087
Commit-Queue: Derek Cheng <imcheng@chromium.org>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560905}
10 files changed