Prepare for |is_posix| switch in the Fuchsia build

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

Bug: chromium:812974
Change-Id: I7028fe83d05a46af7186fe5b1b963016fd35f334
Reviewed-on: https://webrtc-review.googlesource.com/65781
Reviewed-by: Raymond Toy <rtoy@google.com>