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This CL includes concrete implementations for the abstract classes
AtomicBoolean and AtomicReference defined in the Nearby library. An
AtomicBoolean is a boolean value that may be updated atomically, and an
AtomicReference is an object reference that may be updated atomically.

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 (in-progress CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1113198).

Bug: 861813
Change-Id: Ibbda1bd35e3fb63eed0ae492ecebb3d61fa43050
Reviewed-on: https://chromium-review.googlesource.com/1142478
Commit-Queue: Kyle Qian <kyleqian@google.com>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584564}