Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

mojo/common/values_struct_traits.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 MOJO_COMMON_VALUES_STRUCT_TRAITS_H_
 #define MOJO_COMMON_VALUES_STRUCT_TRAITS_H_
 
 #include "base/values.h"
 #include "mojo/common/values.mojom.h"
 #include "mojo/public/cpp/bindings/array_traits.h"
@@ skipping 234 matching lines @@
   static const base::DictionaryValue& dictionary_value(
       const std::unique_ptr<base::Value>& value) {
     return UnionTraits<common::mojom::ValueDataView,
                        base::Value>::dictionary_value(*value);
   }
 
   static bool Read(common::mojom::ValueDataView data,
                    std::unique_ptr<base::Value>* value);
 };
 
+template <>
+struct CloneTraits<std::unique_ptr<base::Value>, false> {
+  static std::unique_ptr<base::Value> Clone(
+      const std::unique_ptr<base::Value>& input);
+};
+
 }  // namespace mojo
 
 #endif  // MOJO_COMMON_VALUES_STRUCT_TRAITS_H_