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macOS has futex()-like locks in pthread, meaning that SpinningFutex can
be useful there, the same way it is on Windows. Use pthread to implement
it.

Bug: 998048
Change-Id: I77df835baf2f099eedcfcee675f7a56ed6b3bb2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2550797Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830175}