Allow building mojo_shell and non-graphical apps/services on a Mac

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 6f8bbd021d28cd612d4e887cf65eac6416bd1fa3..3ae3930531c1f15c23c358041591396091a35073 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -17,8 +17,6 @@ group("default") {
     "//mojom",
     "//services",
     "//shell",
-    "//ui/gfx:gfx_unittests",
-    "//ui/events:events_unittests",
   ]
 
   if (is_linux) {
@@ -29,6 +27,13 @@ group("default") {
     ]
   }
 
+  if (is_linux || is_android) {
+    deps += [
+      "//ui/events:events_unittests",
+      "//ui/gfx:gfx_unittests",
+    ]
+  }
+
   if (use_ozone) {
     deps += [
       "//ui/ozone",