[CAF MR] Implementing CafMessageHandler

This CL migrates the CastMessageHandler for CAF. There's not
much code change between CastMessageHandler and CafMessageHandler.

Bug: 711860
Change-Id: Id58454c51cd6ada50b507d750a979ffe4e248241
Reviewed-on: https://chromium-review.googlesource.com/1169920
Commit-Queue: Zhiqiang Zhang <zqzhang@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583043}
14 files changed