Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

services/preferences/public/cpp/pref_store_client_mixin.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_CLIENT_MIXIN_H_
 #define SERVICES_PREFERENCES_PUBLIC_CPP_PREF_STORE_CLIENT_MIXIN_H_
 
 #include <memory>
 #include <string>
+#include <vector>
 
 #include "base/macros.h"
 #include "base/observer_list.h"
 #include "mojo/public/cpp/bindings/binding.h"
 #include "services/preferences/public/interfaces/preferences.mojom.h"
 
 namespace base {
 class DictionaryValue;
 class Value;
 }
@@ skipping 31 matching lines @@
 
   base::DictionaryValue& GetMutableValues();
   void ReportPrefValueChanged(const std::string& key);
 
  private:
   // prefs::mojom::PreferenceObserver:
   void OnPrefsChanged(std::vector<mojom::PrefUpdatePtr> updates) override;
   void OnInitializationCompleted(bool succeeded) override;
 
   void OnPrefChanged(const std::string& key,
-                     std::unique_ptr<base::Value> value);
+                     mojom::PrefUpdateValuePtr update_value);
 
   // Cached preferences.
   // If null, indicates that initialization failed.
   std::unique_ptr<base::DictionaryValue> cached_prefs_;
 
   base::ObserverList<PrefStore::Observer, true> observers_;
 
   // Has the PrefStore we're observing been initialized?
   bool initialized_ = false;
 
   mojo::Binding<mojom::PrefStoreObserver> observer_binding_;
 
   DISALLOW_COPY_AND_ASSIGN(PrefStoreClientMixin);
 };
 
 extern template class PrefStoreClientMixin<::PrefStore>;
 extern template class PrefStoreClientMixin<::PersistentPrefStore>;
 
 }  // namespace prefs
 
 #endif  // SERVICES_PREFERENCES_PUBLIC_CPP_PREF_STORE_CLIENT_MIXIN_H_