Pref service: expose all read-only PrefStores through Mojo

services/preferences/public/cpp/preferences_struct_traits_macros.h

Index: services/preferences/public/cpp/preferences_struct_traits_macros.h
diff --git a/services/preferences/public/cpp/preferences_struct_traits_macros.h b/services/preferences/public/cpp/preferences_struct_traits_macros.h
new file mode 100644
index 0000000000000000000000000000000000000000..9113580f78aafc7744a1fc320e07657453bb5f40
--- /dev/null
+++ b/services/preferences/public/cpp/preferences_struct_traits_macros.h
@@ -0,0 +1,15 @@
+// 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_MACROS_H_
+#define SERVICES_PREFERENCES_PUBLIC_CPP_PREFERENCES_STRUCT_TRAITS_MACROS_H_
+
+#include "components/prefs/pref_value_store.h"
+#include "ipc/ipc_message_macros.h"
+
+IPC_ENUM_TRAITS_MIN_MAX_VALUE(PrefValueStore::PrefStoreType,
+                              PrefValueStore::INVALID_STORE,
+                              PrefValueStore::PREF_STORE_TYPE_MAX);
+
+#endif  // SERVICES_PREFERENCES_PUBLIC_CPP_PREFERENCES_STRUCT_TRAITS_MACROS_H_