sync: remove use of protobuf extensions in protocol to reduce static init overhead.

chrome/browser/sync/engine/syncer_proto_util.cc

Index: chrome/browser/sync/engine/syncer_proto_util.cc
diff --git a/chrome/browser/sync/engine/syncer_proto_util.cc b/chrome/browser/sync/engine/syncer_proto_util.cc
index 53c917af49c17ebb98e8d8314cfa00b08fc997be..d0712b51fb94fd313db2f37b770261abe2e2d9cf 100644
--- a/chrome/browser/sync/engine/syncer_proto_util.cc
+++ b/chrome/browser/sync/engine/syncer_proto_util.cc
@@ -111,7 +111,7 @@ void SyncerProtoUtil::HandleMigrationDoneResponse(
       << "MIGRATION_DONE but no types specified.";
   syncable::ModelTypeSet to_migrate;
   for (int i = 0; i < response->migrated_data_type_id_size(); i++) {
-    to_migrate.Put(syncable::GetModelTypeFromExtensionFieldNumber(
+    to_migrate.Put(syncable::GetModelTypeFromSpecificsFieldNumber(
         response->migrated_data_type_id(i)));
   }
   // TODO(akalin): This should be a set union.
@@ -301,7 +301,7 @@ browser_sync::SyncProtocolError ConvertErrorPBToLocalType(
     DCHECK_EQ(error.error_type(), sync_pb::SyncEnums::THROTTLED);
     for (int i = 0; i < error.error_data_type_ids_size(); ++i) {
       sync_protocol_error.error_data_types.Put(
-          syncable::GetModelTypeFromExtensionFieldNumber(
+          syncable::GetModelTypeFromSpecificsFieldNumber(
               error.error_data_type_ids(i)));
     }
   }