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This injects a ClientChannel into Messenger, which is used if the
chromeos::features::kMultiDeviceApi is enabled. The ClientChannel is used
to send and receive messages with the remote device.

R=jhawkins@chromium.org, khorimoto@chromium.org

Bug: 824568, 752273
Change-Id: If927eb84d013657db823f8538d138beb9c2c16be
Reviewed-on: https://chromium-review.googlesource.com/1107482
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569798}