Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/nigori_specifics.proto

 // Copyright (c) 2012 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.
 //
 // Sync protocol datatype extension for nigori keys.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
-option retain_unknown_fields = true;
 
 package sync_pb;
 
 import "encryption.proto";
 
 message NigoriKey {
   optional string name = 1;
   optional bytes user_key = 2;
   optional bytes encryption_key = 3;
   optional bytes mac_key = 4;
@@ skipping 110 matching lines @@
 
   // Boolean indicating whether this node was originally missing a
   // |keystore_migration_time| field value, and was fixed on the server by
   // giving the field a value.
   // THIS FIELD SHOULD ONLY BE SET BY THE SERVER.
   optional bool server_only_was_missing_keystore_migration_time = 40;
 
   // Boolean corresponding to whether arc pakcage items should be encrypted.
   optional bool encrypt_arc_package = 41;
 }