[remoting][FTL] Implement ReceiveMessages stream

This CL implements the ReceiveMessages stream and allows
FtlSignalingPlayground to read from the stream.

Bug: 927962
Change-Id: I4c3b155ccf0f4aa0a5f6b241dd8763cb9fd9f928
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1510052
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643513}
16 files changed