Fix iOS build Define PosixSpawnFileActions::Chdir() only on MacOS, similarly to crrev.com/c/4300129. Bug: chromium:1423136 Change-Id: I51055fb07166ecd6f8140afba421a3740fb5eafb Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4448607 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Commit-Queue: Yuly Novikov <ynovikov@chromium.org> Reviewed-by: Scott Violet <sky@chromium.org>