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

|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: I25c9a760ea6176935cb5bf946a9c5b499d93e640
Reviewed-on: https://chromium-review.googlesource.com/988793Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548168}

gpu/BUILD.gn

@@ -425,7 +425,7 @@ test("gpu_unittests") {
     ]
   }
 
-  if (is_posix && !is_mac) {
+  if (is_fuchsia || (is_posix && !is_mac)) {
     sources += [ "command_buffer/service/gpu_fence_manager_unittest.cc" ]
   }