Make PrefService available in *ash

ash/test/BUILD.gn

 # Copyright 2016 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("//build/config/ui.gni")
 
 static_library("test_support_without_content") {
   testonly = true
   sources = [
     "ash_test_environment_default.cc",
@@ skipping 140 matching lines @@
   ]
   deps = [
     "//ash",
     "//ash/mus:lib",
     "//ash/public/cpp:ash_public_cpp",
     "//ash/resources",
     "//base",
     "//base:i18n",
     "//base/test:test_support",
     "//chromeos",
+    "//components/prefs:test_support",
     "//components/signin/core/account_id",
     "//components/user_manager:user_manager",
     "//device/bluetooth",
     "//services/ui/public/interfaces",
     "//skia",
     "//testing/gtest",
     "//ui/accessibility",
     "//ui/app_list:test_support",
     "//ui/app_list/presenter",
     "//ui/app_list/presenter:test_support",
@@ skipping 34 matching lines @@
   deps = [
     ":test_support_with_content",
     "//base",
     "//skia",
     "//testing/gtest",
     "//ui/aura",
     "//ui/base",
     "//ui/gl:test_support",
   ]
 }