Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/arc_package_specifics.proto

 // Copyright 2016 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 ARC pakcages.
 
 // Update proto_{value,enum}_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;
 
 // Properties of arc package objects.
 message ArcPackageSpecifics {
   // Unique identifier for the item:
   // Android package name.
   optional string package_name = 1;
   // Android package version.
   optional int32 package_version = 2;
   // Last Android id generated when Android side backups data.
   optional int64 last_backup_android_id = 3;
   // Last time internal value that Android side backups data.
   optional int64 last_backup_time = 4;
 }