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* Implement a MuxingSignalStrategy to be used exclusively by
  JingleSessionManager to handle messages from both signal strategies.
* Integrate all the signaling stuff into remoting_me2me_host, which can
  be enabled by passing a `--enable-ftl-signaling` flag.

This has been tested with ftl_signaling_playground and I'm able to
finish candidate exchange for ~99% of the attempt. I did see one DCHECK
failure when JingleSession receives a duplicated message, but that will
be tracked and fixed in a different bug/CL.

Bug: 947337
Change-Id: Id6a6a183041a2668a0dd31deb735b01ceb4c4aec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1565537
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652608}