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This cleans up multi-platform code paths selection and prepares for
the removal of OS_POSIX for the Fuchsia build.

Bug: 836416
Change-Id: If44f30dd222572fea1c030a804507f67a1b54e17
Reviewed-on: https://chromium-review.googlesource.com/1033967Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556878}