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This CL changes the constructor for CountDownLatchImpl to use a DCHECK
to disallow initialization counts less than 0. An initialization count
of exactly 0 is still allowed, in which case countDown() will never
Signal() its WaitableEvent, and all calls to Await() will immediately
return.

Bug: 861813
Change-Id: Ic7807544c792be010f46ca58391a02f64ab1638e
Reviewed-on: https://chromium-review.googlesource.com/1141101
Commit-Queue: Kyle Qian <kyleqian@google.com>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576144}