Set |posix| to false for the Fuchsia build.

This follows the chromium change for consistency.

Bug: chromium:812974
Change-Id: I1a831219ed02bbe11cc037bebd4a98d2d7d76739
Reviewed-on: https://chromium-review.googlesource.com/1016034
Reviewed-by: Scott Graham <scottmg@chromium.org>

build/platform.gni

diff --git a/build/platform.gni b/build/platform.gni
index 16bf69a..bef5254 100644
--- a/build/platform.gni
+++ b/build/platform.gni
@@ -21,4 +21,4 @@
 }
 
 mini_chromium_is_posix = mini_chromium_is_mac || mini_chromium_is_linux ||
-                         mini_chromium_is_android || mini_chromium_is_fuchsia
+                         mini_chromium_is_android