Add struct traits for base::Value.

services/preferences/public/cpp/pref_observer_store.h

 // 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.
 
 #ifndef SERVICES_PREFERENCES_PUBLIC_CPP_PREFS_OBSERVER_STORE_H_
 #define SERVICES_PREFERENCES_PUBLIC_CPP_PREFS_OBSERVER_STORE_H_
 
 #include <set>
 
 #include "base/macros.h"
@@ skipping 43 matching lines @@
   ~PrefObserverStore() override;
 
  private:
   friend class PrefObserverStoreTest;
 
   // Notifies |prefs_manager_| of the change in |key| - |value| pair.
   void SetValueOnPreferenceManager(const std::string& key,
                                    base::Value const & value);
 
   // prefs::mojom::PreferenceObserver:
-  void OnPreferencesChanged(const base::DictionaryValue& preferences) override;
+  void OnPreferencesChanged(
+      std::unique_ptr<base::DictionaryValue> preferences) override;
 
   mojo::Binding<prefs::mojom::PreferencesObserver> prefs_binding_;
   prefs::mojom::PreferencesManagerPtr prefs_manager_ptr_;
 
   std::set<std::string> keys_;
 
   // True upon the first OnPreferencesChanged received after Init.
   bool initialized_;
 
   DISALLOW_COPY_AND_ASSIGN(PrefObserverStore);
 };
 
 #endif  // SERVICES_PREFERENCES_PUBLIC_CPP_PREFS_OBSERVER_STORE_H_