Add chrome test support target to GN build.

components/pref_registry/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.
 
 component("pref_registry") {
   sources = [
     "pref_registry_export.h",
     "pref_registry_syncable.cc",
     "pref_registry_syncable.h",
   ]
 
   defines = [
     "PREF_REGISTRY_IMPLEMENTATION",
   ]
 
   deps = [
     "//base",
     "//base:prefs",
     "//base/third_party/dynamic_annotations",
     "//ui/base",
   ]
 }
 
-source_set("pref_registry_test_support") {
+source_set("test_support") {
+  testonly = true
   sources = [
     "testing_pref_service_syncable.cc",
     "testing_pref_service_syncable.h",
   ]
 
   deps = [
     ":pref_registry",
   ]
 }