Reland "Fuchsia: Stop setting OS_POSIX"

This is a reland of 90f86c118d322a2ad437157a71cef8fc43a17d01

Original change's description:
> Fuchsia: Stop setting OS_POSIX
>
> Bug: 836416
> Change-Id: Ib59796358ff2109ec0b55686974a84980e18254e
> Reviewed-on: https://chromium-review.googlesource.com/1020375
> Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Reviewed-by: Scott Graham <scottmg@chromium.org>
> Reviewed-by: Matt Mueller <mattm@chromium.org>
> Reviewed-by: Victor Costan <pwnall@chromium.org>
> Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#560013}

Bug: 836416
Change-Id: I43f34aa644e51d231c7c131b4be364b6ccee2d38
Reviewed-on: https://chromium-review.googlesource.com/1066711
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#560828}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 65421f69a6e0e69991c3d6521f98efa8647b547f

build_config.h

@@ -79,10 +79,10 @@
 
 // For access to standard POSIXish features, use OS_POSIX instead of a
 // more specific macro.
-#if defined(OS_AIX) || defined(OS_ANDROID) || defined(OS_ASMJS) ||     \
-    defined(OS_FREEBSD) || defined(OS_FUCHSIA) || defined(OS_LINUX) || \
-    defined(OS_MACOSX) || defined(OS_NACL) || defined(OS_NETBSD) ||    \
-    defined(OS_OPENBSD) || defined(OS_QNX) || defined(OS_SOLARIS)
+#if defined(OS_AIX) || defined(OS_ANDROID) || defined(OS_ASMJS) ||    \
+    defined(OS_FREEBSD) || defined(OS_LINUX) || defined(OS_MACOSX) || \
+    defined(OS_NACL) || defined(OS_NETBSD) || defined(OS_OPENBSD) ||  \
+    defined(OS_QNX) || defined(OS_SOLARIS)
 #define OS_POSIX 1
 #endif
 
@@ -179,6 +179,8 @@
 // Type detection for wchar_t.
 #if defined(OS_WIN)
 #define WCHAR_T_IS_UTF16
+#elif defined(OS_FUCHSIA)
+#define WCHAR_T_IS_UTF32
 #elif defined(OS_POSIX) && defined(COMPILER_GCC) && defined(__WCHAR_MAX__) && \
     (__WCHAR_MAX__ == 0x7fffffff || __WCHAR_MAX__ == 0xffffffff)
 #define WCHAR_T_IS_UTF32