Pref service: add persistent pref store frontend and backend.

services/preferences/public/cpp/preferences_struct_traits.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_PREFERENCES_STRUCT_TRAITS_H_
 #define SERVICES_PREFERENCES_PUBLIC_CPP_PREFERENCES_STRUCT_TRAITS_H_
 
+#include "components/prefs/persistent_pref_store.h"
 #include "components/prefs/pref_value_store.h"
 #include "mojo/public/cpp/bindings/enum_traits.h"
 #include "services/preferences/public/interfaces/preferences.mojom-shared.h"
 
 namespace mojo {
 
 template <>
 struct EnumTraits<::prefs::mojom::PrefStoreType,
                   ::PrefValueStore::PrefStoreType> {
   static prefs::mojom::PrefStoreType ToMojom(
       PrefValueStore::PrefStoreType input);
 
   static bool FromMojom(prefs::mojom::PrefStoreType input,
                         PrefValueStore::PrefStoreType* output);
 };
 
+template <>
+struct EnumTraits<::prefs::mojom::PersistentPrefStoreConnector_ReadError,
+                  ::PersistentPrefStore::PrefReadError> {
+  static prefs::mojom::PersistentPrefStoreConnector_ReadError ToMojom(
+      PersistentPrefStore::PrefReadError input);
+
+  static bool FromMojom(
+      prefs::mojom::PersistentPrefStoreConnector_ReadError input,
+      PersistentPrefStore::PrefReadError* output);
+};
+
 }  // namespace mojo
 
 #endif  // SERVICES_PREFERENCES_PUBLIC_CPP_PREFERENCES_STRUCT_TRAITS_H_