GitLab

- ConnectionManager exposes SendMessage to send a message
  through the client channel.
- Adds the OnMessageReceived() to ConnectionManager's observer
  to notify clients of new messages from the client channel.

Bug: 1106937
Test: unit_tests
Change-Id: I9b2de4c7c05855deaf1ea8cebf2d01b2d4e3c69c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2390972
Commit-Queue: Jimmy Gong <jimmyxgong@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805085}