Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/managed_user_specifics.proto

 // Copyright 2013 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 managed user settings.
 
 // 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;
 
 // Properties of managed user sync objects.
 message ManagedUserSpecifics {
   // A randomly-generated identifier for the managed user.
   optional string id = 1;
   // The human-visible name of the managed user
   optional string name = 2;
   // This flag is set by the server to acknowledge that it has committed a
   // newly created managed user.
   optional bool acknowledged = 3 [default = false];
   // Master key for managed user cryptohome.
   optional string master_key = 4;
   // A string representing the index of the supervised user avatar on Chrome.
   // It has the following format:
   // "chrome-avatar-index:INDEX" where INDEX is an integer.
   optional string chrome_avatar = 5;
   // A string representing the index of the supervised user avatar on Chrome OS.
   // It has the following format:
   // "chromeos-avatar-index:INDEX" where INDEX is an integer.
   optional string chromeos_avatar = 6;
   // Key for signing supervised user's password.
   optional string password_signature_key = 7;
   // Key for encrypting supervised user's password.
   optional string password_encryption_key = 8;
 }