Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

components/prefs/BUILD.gn

 # Copyright 2015 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("prefs") {
   sources = [
     "base_prefs_export.h",
     "command_line_pref_store.cc",
     "command_line_pref_store.h",
     "default_pref_store.cc",
@@ skipping 25 matching lines @@
     "pref_store.cc",
     "pref_store.h",
     "pref_value_map.cc",
     "pref_value_map.h",
     "pref_value_store.cc",
     "pref_value_store.h",
     "scoped_user_pref_update.cc",
     "scoped_user_pref_update.h",
     "value_map_pref_store.cc",
     "value_map_pref_store.h",
+    "writeable_pref_store.cc",
     "writeable_pref_store.h",
   ]
 
   defines = [ "COMPONENTS_PREFS_IMPLEMENTATION" ]
 
   deps = [
     "//base",
   ]
 }
 
@@ skipping 37 matching lines @@
   ]
 
   deps = [
     ":test_support",
     "//base",
     "//base/test:test_support",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }