GitLab

Add MessageDispatcher to handle sending/receiving cast messages.

Bug: 734672
Change-Id: I1941ba72107b4629da8c47f0b726484d800b2891
Reviewed-on: https://chromium-review.googlesource.com/1008729
Commit-Queue: Xiangjun Zhang <xjz@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555622}