Pref service: expose all read-only PrefStores through Mojo

services/preferences/public/cpp/preferences_struct_traits.h

Index: services/preferences/public/cpp/preferences_struct_traits.h
diff --git a/services/preferences/public/cpp/preferences_struct_traits.h b/services/preferences/public/cpp/preferences_struct_traits.h
new file mode 100644
index 0000000000000000000000000000000000000000..d548d8faa38017e41d7e8f45db26cdf7b4cdedbc
--- /dev/null
+++ b/services/preferences/public/cpp/preferences_struct_traits.h
@@ -0,0 +1,26 @@
+// 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/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);
+};
+
+}  // namespace mojo
+
+#endif  // SERVICES_PREFERENCES_PUBLIC_CPP_PREFERENCES_STRUCT_TRAITS_H_