Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/bookmark_specifics.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 datatype extension for bookmarks.
 
 // 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;
 
 // Corresponds to a single meta info key/value pair for a bookmark node.
 message MetaInfo {
   optional string key = 1;
   optional string value = 2;
 }
 
 // Properties of bookmark sync objects.
 message BookmarkSpecifics {
   optional string url = 1;
   optional bytes favicon = 2;
   optional string title = 3;
   // Corresponds to BookmarkNode::date_added() and is the internal value from
   // base::Time.
   optional int64 creation_time_us = 4;
   optional string icon_url = 5;
   repeated MetaInfo meta_info = 6;
 }