Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/client_commands.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;
 
 message CustomNudgeDelay {
   optional int32 datatype_id = 1;  // Datatype id.
   optional int32 delay_ms = 2;     // Delay in milliseconds.
 };
 
 message ClientCommand {
   // Time to wait before sending any requests to the server.
@@ skipping 14 matching lines @@
 
   // Time to wait before issuing a retry GU.
   optional int32 gu_retry_delay_seconds = 7;
 
   // A dictionary of custom nudge delays.
   // Note: if a SESSIONS value is present, this will override
   // |sessions_commit_delay_seconds|
   // New in M39.
   repeated CustomNudgeDelay custom_nudge_delays = 8;
 };