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This injects a ClientChannel into ProximityAuthMonitor, which is used if the
chromeos::features::kMultiDeviceApi is enabled. The ClientChannel is used
to get the current RSSI of the connected remote device.

In the future, the "rolling average RSSI" that is calculated in
ProximityAuthMonitor will be moved to the SecureChannel API, and returned
by it. However, to reduce immediate migration work, that logic is kept
in ProximityAuthMonitor for now.

This CL was originally submitted as [1], but was reverted [2] due to a
subtle bug in a test, which has been fixed at [3]. This CL is unchanged
from the original.

1) https://chromium-review.googlesource.com/c/chromium/src/+/1106616
2) https://chromium-review.googlesource.com/c/chromium/src/+/1111937
3) https://chromium-review.googlesource.com/c/chromium/src/+/1112422

Bug: 824568, 752273
Change-Id: I5fba2dcb73fd4b8da08d93cbfa045b6f6888a93b
Reviewed-on: https://chromium-review.googlesource.com/1112434
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569785}