|is_posix| will be switched to false for Fuchsia, this is a preliminary change.

Bug: chromium:812974
Change-Id: I2ae8e5ef4270cd0acbc7c69bb46d1be9ed4a5c5e
Reviewed-on: https://chromium-review.googlesource.com/988253Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52407}