Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

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/catalog/public/tools/catalog.gni")
 import("//services/service_manager/public/service_manifest.gni")
 import("//testing/test.gni")
 
 service_manifest("manifest") {
   name = "preferences"
   source = "manifest.json"
 }
 
 source_set("preferences") {
   visibility = [
     ":*",
     "//services/preferences/public/cpp:service_main",
   ]
   deps = [
     "//components/prefs",
     "//services/preferences/public/cpp",
+    "//services/preferences/public/cpp/lib",
     "//services/preferences/public/interfaces",
     "//services/preferences/tracked",
     "//services/service_manager/public/cpp",
   ]
   sources = [
     "persistent_pref_store_factory.cc",
     "persistent_pref_store_factory.h",
     "persistent_pref_store_impl.cc",
     "persistent_pref_store_impl.h",
     "pref_store_manager_impl.cc",
@@ skipping 30 matching lines @@
 service_manifest("unittest_manifest") {
   name = "prefs_unittests"
   source = "unittest_manifest.json"
   packaged_services = [ ":manifest" ]
 }
 
 catalog("tests_catalog") {
   testonly = true
   embedded_services = [ ":unittest_manifest" ]
 }