Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/get_updates_caller_info.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.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
-option retain_unknown_fields = true;
 
 package sync_pb;
 
 message GetUpdatesCallerInfo {
   // This message was deprecated in M28.  The preferred represenation of this
   // information is now the GetUpdatesOrigin enum, which is defined in
   // sync_enums.proto.
   enum GetUpdatesSource {
     UNKNOWN = 0;       // The source was not set by the caller.
     FIRST_UPDATE = 1;  // First request after browser restart.  Not to
@@ skipping 26 matching lines @@
     RETRY = 13;  // A follow-up GU to pick up updates missed by previous GU.
     PROGRAMMATIC = 14;  // The client is programmatically enabling/disabling
                         // a type, typically for error handling purposes.
   }
 
   required GetUpdatesSource source = 1;
 
   // True only if notifications were enabled for this GetUpdateMessage.
   optional bool notifications_enabled = 2;
 };