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This is a reland of 2096391c
Original change's description:
> Reimplement base::WaitableEvent with a kqueue on Mac.
> 
> For a single WaitableEvent, a custom EVFILT_USER kevent is used to wait and
> signal. This replaces the default POSIX implementation that uses a
> pthread_cond_t and a boolean flag.
> 
> To implement WaitMany, a new kqueue is created to wait on all the individual
> WaitableEvent's kqueue descriptor. This replaces a complex locking algorithm
> used in the default POSIX implementation.
> 
> For the asynchronous WaitableEventWatcher, a TYPE_READ dispatch_source_t is
> used to watch the WaitableEvent's kqueue. This replaces the POSIX
> implementation of a reference-counted list of async watchers guarded by a lock.
> 
> Microbenchmarks show that the kqueue implementation is significantly faster in
> most cases. The one potential drawback is hitting the low RLIMIT_NOFILE on
> macOS, since each WaitableEvent and WaitableEventWatcher requires a new
> descriptor.
> 
> Bug: 681167
> Change-Id: I135012fdd25e547ffb911fc7adc97c203df38241
> Reviewed-on: https://chromium-review.googlesource.com/553497
> Reviewed-by: Robert Liao <robliao@chromium.org>
> Reviewed-by: Mark Mentovai <mark@chromium.org>
> Commit-Queue: Robert Sesek <rsesek@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#485788}

Bug: 681167
Change-Id: I1b0296022f7bd1a31b074fbbb551bccc3b28d05b
Reviewed-on: https://chromium-review.googlesource.com/568642Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486449}