Revert "Fuchsia: Stop setting OS_POSIX"

This reverts commit 90f86c118d322a2ad437157a71cef8fc43a17d01.

Reason for revert: Broke builders
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Fuchsia%20ARM64%20Cast%20Audio/14741
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Fuchsia%20x64%20Cast%20Audio/14712

Original change's description:
> Fuchsia: Stop setting OS_POSIX
> 
> Bug: 836416
> Change-Id: Ib59796358ff2109ec0b55686974a84980e18254e
> Reviewed-on: https://chromium-review.googlesource.com/1020375
> Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Reviewed-by: Scott Graham <scottmg@chromium.org>
> Reviewed-by: Matt Mueller <mattm@chromium.org>
> Reviewed-by: Victor Costan <pwnall@chromium.org>
> Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#560013}

TBR=kinuko@chromium.org,thakis@chromium.org,mattm@chromium.org,fdegans@chromium.org,scottmg@chromium.org,pwnall@chromium.org

Change-Id: I6f8a28690dddefe8acf99220646def362bdc9bc8
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 836416
Reviewed-on: https://chromium-review.googlesource.com/1066332
Reviewed-by: Fabrice de Gans-Riberi <fdegans@chromium.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560031}

CrOS-Libchrome-Original-Commit: bd1301fffdda4fcb3117379788f767b0c42e85d1
11 files changed
tree: 54aec391612cf8d8cf8c9e42ce817b28d3c70052
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/