Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/unique_position.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.
 //
 // Protobuf representation of the UniquePosition class.
 
 // 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;
 
 // A UniquePosition is a string of bytes.
 //
 // Unique positions are unique per-item, since they are guaranteed to end with a
 // fixed-length suffix that is unique per-item.  The position string may not end
 // with a '\0' byte.
 //
 // Prior to the suffix is a series of arbitrary bytes of arbitrary length.
@@ skipping 47 matching lines @@
   // compressed in memory.
   //
   // The compression scheme is implemented and documented in
   // sync/core_impl/base/unique_position.cc.
   //
   // As of M30, this is the preferred encoding.  Newer clients may continue to
   // populate the 'value' and 'compressed_value' fields to ensure backwards
   // compatibility, but they will always try to read from this field first.
   optional bytes custom_compressed_v1 = 4;
 }