Fuchsia: Prepare for OS_POSIX removal in base/

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

Bug: 836416
Change-Id: I3a8e6393d0a9981aa890fb6d8c302ed8d2bdd69d
Reviewed-on: https://chromium-review.googlesource.com/1031099
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559230}
51 files changed