GitLab

This CL includes a concrete implementation for the abstract classes
Lock (which must be recursive) and ConditionVariable defined in the
Nearby library.

A related CL was previously abandoned
(https://chromium-review.googlesource.com/c/chromium/src/+/1138871)
because the Nearby C++ library already includes a DefaultLock and
DefaultConditionVariable. However, that CL is being revived as this CL
because a Chromium-specific Lock/CV implementation is required. For
instance, Chromium requires a ScopedBlockingCall to be instantiated in
scopes where blocking calls are made. DefaultConditionVariable does not
include an instantiation of ScopedBlockingCall. This implementation of
ConditionVariable uses a base::WaitableEvent, which instantiates a
ScopedBlockingCall. This CL also includes a Lock implementation that is
recursive, which is a requirement of Nearby, but is not originally
supported by base::Lock.

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: I72660ed302e239d8f38a21728fc715b000057c13
Reviewed-on: https://chromium-review.googlesource.com/1145730
Commit-Queue: Kyle Qian <kyleqian@google.com>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586070}