Pref service: expose all read-only PrefStores through Mojo

services/preferences/public/cpp/tests/BUILD.gn

 # 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.
 
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 
 group("tests") {
   testonly = true
   deps = [
     ":preferences_unittests",
   ]
 }
 
 test("preferences_unittests") {
   sources = [
     "pref_client_store_unittest.cc",
+    "pref_store_client_unittest.cc",
   ]
   deps = [
     "//base",
     "//base/test:test_support",
     "//components/prefs:test_support",
     "//mojo/edk/test:run_all_unittests",
     "//mojo/public/cpp/bindings:bindings",
     "//services/preferences/public/cpp",
     "//services/preferences/public/interfaces",
+    "//testing/gmock",
     "//testing/gtest",
   ]
 }