Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

mojo/common/values_struct_traits.h

Index: mojo/common/values_struct_traits.h
diff --git a/mojo/common/values_struct_traits.h b/mojo/common/values_struct_traits.h
index 7d8c2ed72171c09e1c76170fd1b38deac40a6441..d5024010da5fc804e1cec866867cf77877160614 100644
--- a/mojo/common/values_struct_traits.h
+++ b/mojo/common/values_struct_traits.h
@@ -252,6 +252,12 @@ struct UnionTraits<common::mojom::ValueDataView, std::unique_ptr<base::Value>> {
                    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_