[iOS/GN] Fix the compilation of "all" on iOS with gn.

mojo/edk/test/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//testing/test.gni")
 
 source_set("test_support") {
   testonly = true
   sources = [
     "mojo_test_base.cc",
     "mojo_test_base.h",
-    "multiprocess_test_helper.cc",
-    "multiprocess_test_helper.h",
     "scoped_ipc_support.cc",
     "scoped_ipc_support.h",
     "test_utils.h",
     "test_utils_posix.cc",
     "test_utils_win.cc",
   ]
 
+  if (!is_ios) {
+    sources += [
+      "multiprocess_test_helper.cc",
+      "multiprocess_test_helper.h",
+    ]
+  }
+
   deps = [
     "//base",
     "//base/test:test_support",
     "//mojo/edk/system",
     "//mojo/public/cpp/system",
     "//testing/gtest",
   ]
 }
 
 source_set("run_all_unittests") {
@@ skipping 88 matching lines @@
     "//mojo/public/c/system/tests:perftests",
   ]
 }
 
 test("mojo_public_system_unittests") {
   deps = [
     ":run_all_unittests",
     "//mojo/public/cpp/system/tests",
   ]
 }