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

services/preferences/public/cpp/pref_registry_serializer.cc

Index: services/preferences/public/cpp/pref_registry_serializer.cc
diff --git a/services/preferences/public/cpp/pref_registry_serializer.cc b/services/preferences/public/cpp/pref_registry_serializer.cc
index b64e2c787d49fbdfec4890d91275e78fb7b7fe42..9f8d3de5585c78dba8298461cafbd75d340502aa 100644
--- a/services/preferences/public/cpp/pref_registry_serializer.cc
+++ b/services/preferences/public/cpp/pref_registry_serializer.cc
@@ -12,8 +12,7 @@ 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;
 }