Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

services/preferences/public/cpp/pref_store_impl.h

 // Copyright 2017 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.
 
 #ifndef SERVICES_PREFERENCES_PUBLIC_CPP_PREF_STORE_IMPL_H_
 #define SERVICES_PREFERENCES_PUBLIC_CPP_PREF_STORE_IMPL_H_
 
+#include <string>
 #include <vector>
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "components/prefs/pref_store.h"
 #include "components/prefs/pref_value_store.h"
 #include "mojo/public/cpp/bindings/binding.h"
 #include "services/preferences/public/interfaces/preferences.mojom.h"
 
 namespace prefs {
@@ skipping 36 matching lines @@
   bool backing_pref_store_initialized_;
 
   mojo::Binding<mojom::PrefStore> binding_;
 
   DISALLOW_COPY_AND_ASSIGN(PrefStoreImpl);
 };
 
 }  // namespace prefs
 
 #endif  // SERVICES_PREFERENCES_PUBLIC_CPP_PREF_STORE_IMPL_H_