Pref service: Add initial support for pref registration.

services/preferences/public/cpp/pref_registry_serializer.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_registry_serializer.h"
+
+#include "components/prefs/pref_registry.h"
+
+namespace prefs {
+
+mojom::PrefRegistryPtr SerializePrefRegistry(PrefRegistry& pref_registry) {
+  auto registry = mojom::PrefRegistry::New();
+  for (auto& pref : pref_registry) {
+    registry->registrations[pref.first] = mojom::PrefRegistration::New(
+        pref.second->CreateDeepCopy(),
+        pref_registry.GetRegistrationFlags(pref.first));
+  }
+  return registry;
+}
+
+}  // namespace prefs