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

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

Bug: 812974
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I884fbeef63c4566ecbcaee9fcc5b598af499aef5
Reviewed-on: https://chromium-review.googlesource.com/988953Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548553}

ui/gl/BUILD.gn

@@ -216,7 +216,7 @@ component("gl") {
       }
     }
 
-    if (is_posix) {
+    if (is_posix || is_fuchsia) {
       # Windows has USE_EGL but doesn't support base::FileDescriptor
       sources += [
         "gl_fence_android_native_fence_sync.cc",