Move PrefStoreManagerImpl into //services/preferences.

services/preferences/public/cpp/pref_service_main.cc

+// 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.
+
+#include "services/preferences/public/cpp/pref_service_main.h"
+
+#include "base/threading/sequenced_worker_pool.h"
+#include "services/preferences/pref_store_manager_impl.h"
+#include "services/service_manager/public/cpp/service.h"
+
+namespace prefs {
+
+std::unique_ptr<service_manager::Service> CreatePrefService(
+    std::set<PrefValueStore::PrefStoreType> expected_pref_stores,
+    scoped_refptr<base::SequencedWorkerPool> worker_pool) {
+  return base::MakeUnique<PrefStoreManagerImpl>(expected_pref_stores,
+                                                std::move(worker_pool));
+}
+
+}  // namespace prefs