Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/sync.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 for communication between sync client and server.
 
 // 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 "app_list_specifics.proto";
 import "app_notification_specifics.proto";
 import "app_setting_specifics.proto";
 import "app_specifics.proto";
 import "arc_package_specifics.proto";
 import "article_specifics.proto";
 import "attachments.proto";
@@ skipping 1004 matching lines @@
 message EventResponse {};
 
 // A message indicating that the sync engine has been disabled on a client.
 message SyncDisabledEvent {
   // The GUID that identifies the sync client.
   optional string cache_guid = 1;
 
   // The store birthday that the client was using before disabling sync.
   optional string store_birthday = 2;
 };