Pref service: expose all read-only PrefStores through Mojo

mojo/public/tools/bindings/chromium_bindings_configuration.gni

 # Copyright 2016 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.
 
 _typemap_imports = [
   "//ash/public/interfaces/typemaps.gni",
   "//cc/ipc/typemaps.gni",
   "//chrome/browser/media/router/mojo/typemaps.gni",
   "//chrome/common/extensions/typemaps.gni",
   "//chrome/common/importer/typemaps.gni",
@@ skipping 10 matching lines @@
   "//device/gamepad/public/interfaces/typemaps.gni",
   "//device/generic_sensor/public/interfaces/typemaps.gni",
   "//device/usb/public/interfaces/typemaps.gni",
   "//gpu/ipc/common/typemaps.gni",
   "//media/capture/mojo/typemaps.gni",
   "//media/mojo/interfaces/typemaps.gni",
   "//mojo/common/typemaps.gni",
   "//mojo/public/cpp/bindings/tests/chromium_typemaps.gni",
   "//net/interfaces/typemaps.gni",
   "//services/resource_coordinator/public/cpp/typemaps.gni",
+  "//services/preferences/public/cpp/typemaps.gni",
   "//services/service_manager/public/cpp/typemaps.gni",
   "//services/ui/gpu/interfaces/typemaps.gni",
   "//services/ui/public/interfaces/ime/typemaps.gni",
   "//services/video_capture/public/interfaces/typemaps.gni",
   "//skia/public/interfaces/typemaps.gni",
   "//third_party/WebKit/public/public_typemaps.gni",
   "//ui/base/mojo/typemaps.gni",
   "//ui/display/mojo/typemaps.gni",
   "//ui/events/devices/mojo/typemaps.gni",
   "//ui/events/mojo/typemaps.gni",
@@ skipping 29 matching lines @@
   _typemaps_mac += _imported.typemaps
 }
 
 typemaps_mac = []
 foreach(typemap, _typemaps_mac) {
   typemaps_mac += [ {
         filename = typemap
         config = read_file(typemap, "scope")
       } ]
 }