Switch base::Value typemapping to be by value instead of by unique_ptr.

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));
+        *pref.second, pref_registry.GetRegistrationFlags(pref.first));
   }
   return registry;
 }
 
 }  // namespace prefs