Pref service: Filter updates from read-only pref stores.

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.
 
 source_set("tests") {
   testonly = true
   sources = [
     "pref_client_store_unittest.cc",
     "pref_store_client_unittest.cc",
+    "pref_store_impl_unittest.cc",
   ]
   deps = [
     "//base",
     "//base/test:test_support",
     "//components/prefs:test_support",
     "//mojo/public/cpp/bindings:bindings",
     "//services/preferences/public/cpp",
     "//services/preferences/public/interfaces",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }