Pref service: add support for tracked prefs.

services/preferences/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("//services/service_manager/public/service_manifest.gni")
 import("//testing/test.gni")
 
 group("tests") {
   testonly = true
   deps = [
     ":preferences_unittests",
   ]
 }
 
 service_manifest("manifest") {
   name = "preferences2"
   source = "manifest.json"
 }
 
 source_set("preferences") {
   public_deps = [
     "//services/preferences/public/interfaces",
   ]
   deps = [
     "//components/prefs",
+    "//components/user_prefs/tracked:user_prefs_tracked",
   ]
   sources = [
     "persistent_pref_store_factory.cc",
     "persistent_pref_store_factory.h",
     "persistent_pref_store_impl.cc",
     "persistent_pref_store_impl.h",
   ]
 }
 
 test("preferences_unittests") {
   deps = [
     ":preferences",
     "//base",
     "//base/test:test_support",
     "//components/prefs:test_support",
     "//mojo/edk/test:run_all_unittests",
     "//mojo/public/cpp/bindings:bindings",
     "//services/preferences/public/cpp",
     "//services/preferences/public/cpp/tests",
     "//services/preferences/public/interfaces",
     "//testing/gmock",
     "//testing/gtest",
   ]
   sources = [
     "persistent_pref_store_impl_unittest.cc",
   ]
 }