Prepare for |is_posix| switch in the Fuchsia build in freetype/

|is_posix| will be switched to false for Fuchsia, this is a preliminary change.

Bug: 812974
Change-Id: I930c1866c1ee1121fc08720e94b3075b084ee250
Reviewed-on: https://chromium-review.googlesource.com/988795Reviewed-by: Ben Wagner <bungeman@chromium.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548152}

third_party/freetype/BUILD.gn

@@ -85,7 +85,7 @@ source_set("freetype_source") {
   # While Chromium itself avoids using this code, any build of Chromium which
   # depends on //third_party/fontconfig:fontconfig with use_bundled_fontconfig
   # will make heavy use of these implementations, so use the best available.
-  if (is_posix) {
+  if (is_posix || is_fuchsia) {
     sources += [ "src/builds/unix/ftsystem.c" ]
 
     # builds/unix/ftsystem.c directly includes <ftconfig.h> instead of using