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: I3bfda3e056ad1e5229834286ce5d095d9204a428
Reviewed-on: https://webrtc-review.googlesource.com/65782
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#22753}
diff --git a/BUILD.gn b/BUILD.gn
index 32dc81d..edb033c 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -120,7 +120,7 @@
       ".",
     ]
   }
-  if (is_posix) {
+  if (is_posix || is_fuchsia) {
     defines += [ "WEBRTC_POSIX" ]
   }
   if (is_ios) {
@@ -213,7 +213,7 @@
       "LOGGING_INSIDE_WEBRTC",
     ]
   } else {
-    if (is_posix) {
+    if (is_posix || is_fuchsia) {
       # Enable more warnings: -Wextra is currently disabled in Chromium.
       cflags = [
         "-Wextra",