[remoting][FTL] Implement FtlMessagingClient

This CL moves messaging logic from FtlSignalingPlayground into a new
FtlMessagingClient.

Bug: 927962
Change-Id: I15a8de7da0495bc1b46951e405e47386a1cf27b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1507394
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642758}
8 files changed