Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/managed_user_shared_setting_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 shared 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 shared setting sync objects.
 message ManagedUserSharedSettingSpecifics {
   // The MU ID for the managed user to whom the setting applies.
   optional string mu_id = 1;
   // The key of the setting.
   optional string key = 2;
   // The setting value. The setting is a JSON encoding of an arbitrary
   // Javascript value.
   optional string value = 3;
   // This flag is set by the server to acknowledge that it has committed a
   // change to a setting.
   optional bool acknowledged = 4 [default = false];
 }