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This CL includes a concrete implementation for the abstract class
SystemClock defined in the Nearby library.

SystemClock currently contains a single method elapsedRealtime(),
which is expected to return time in milliseconds since boot, including
time spent in states such as suspension (closed lid). The
implementation in this CL, however, does not include time spent in
suspension due to a current limitation of the POSIX implementation of
base::TimeTicks::Now(). See https://crbug.com/166153 for info.

Because the actual Nearby library has yet to be merged into the CrOS
directory, this CL includes stand-in Nearby abstract classes under
the temporary directory //chromeos/components/nearby/library.
This directory will be removed after the Nearby library gets imported
to //third_party.

Bug: 861813
Change-Id: I194880c75c4a8ba3afa37b6b3c916485c065f20f
Reviewed-on: https://chromium-review.googlesource.com/1150822Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: Kyle Qian <kyleqian@google.com>
Cr-Commit-Position: refs/heads/master@{#579255}