Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

services/preferences/public/cpp/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.
 
 source_set("cpp") {
   sources = [
     "persistent_pref_store_client.cc",
     "persistent_pref_store_client.h",
     "pref_registry_serializer.cc",
     "pref_registry_serializer.h",
     "pref_service_factory.cc",
     "pref_service_factory.h",
     "pref_store_adapter.cc",
     "pref_store_adapter.h",
     "pref_store_client.cc",
     "pref_store_client.h",
     "pref_store_client_mixin.cc",
     "pref_store_client_mixin.h",
     "pref_store_impl.cc",
     "pref_store_impl.h",
+    "scoped_pref_update.cc",
+    "scoped_pref_update.h",
   ]
 
   public_deps = [
     "//base",
     "//components/prefs",
     "//services/preferences/public/interfaces",
     "//services/service_manager/public/cpp",
   ]
 
   deps = [
     "//mojo/public/cpp/bindings",
+    "//services/preferences/public/cpp/lib",
   ]
 }
 
 source_set("service_main") {
   deps = [
     "//base",
     "//components/prefs",
     "//services/preferences",
     "//services/service_manager/public/cpp",
   ]
   sources = [
     "pref_service_main.cc",
     "pref_service_main.h",
   ]
 }