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A WaitableEvent is now a Mach port with a message queue length of one.
Signaling the event enqueues an empty message on the port, or times out
if one is already queued. Waiting on the event receives a message on the
port, dequeuing it if the event is auto-reset, or just peeking it if the
event is manual-reset.

WaitMany is implemented by adding all the events' ports to a port set and
receiving on it.

WaitableEventWatcher is implemented using a TYPE_MACH_RECV dispatch source
for auto-reset events and manual-reset events on 10.12. For manual-reset
events on macOS prior to 10.12, a lock-protected list of watcher callbacks is
used instead of dispatch, because dispatch does not provide timely and
reliable invocation callbacks if the event were to be Reset() immediately after
Signal().

Bug: 681167
Change-Id: I22a9294ad0ae8900d16716d8033285fe91510eda
Reviewed-on: https://chromium-review.googlesource.com/592516
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491860}