Pref service: expose all read-only PrefStores through Mojo

services/preferences/public/cpp/preferences.typemap

Index: services/preferences/public/cpp/preferences.typemap
diff --git a/services/preferences/public/cpp/preferences.typemap b/services/preferences/public/cpp/preferences.typemap
new file mode 100644
index 0000000000000000000000000000000000000000..c15d6123ddd8bde9ca7489636a1698456057c318
--- /dev/null
+++ b/services/preferences/public/cpp/preferences.typemap
@@ -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.
+
+mojom = "//services/preferences/public/interfaces/preferences.mojom"
+public_headers = [ "//components/prefs/pref_value_store.h" ]
+traits_headers =
+    [ "//services/preferences/public/cpp/preferences_struct_traits.h" ]
+sources = [
+  "//services/preferences/public/cpp/preferences_struct_traits.cc",
+]
+deps = [
+  "//components/prefs",
+]
+type_mappings = [ "prefs.mojom.PrefStoreType=::PrefValueStore::PrefStoreType" ]