Clarify a comment about Fuchsia sharing a configuration with POSIX.

It was decided that instances of |posix or fuchsia| in the build or
source files would mean that Fuchsia is explicitly using POSIX
behavior.

Bug: 830987
Test: No-op
Change-Id: If90cd5bfcdba8cd1e60af23e3ede992d9f16fc0b
Reviewed-on: https://chromium-review.googlesource.com/c/1404424
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621788}
1 file changed