Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/extension_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 extensions.
 
 // 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 extension sync objects.
 //
 // Merge policy: the settings for the higher version number win; in
 // the case of a tie, server wins.
 message ExtensionSpecifics {
   // Globally unique id for this extension.
   optional string id = 1;
@@ skipping 23 matching lines @@
   // This is a tri-state boolean, so, unlike most fields here, it really *is*
   // optional and may be absent. We need this for the time being because we need
   // to know if a user has not set an explicit preference.
   optional bool all_urls_enabled = 9;
 
   // Bitmask of the set of reasons why the extension is disabled (see
   // Extension::DisableReason). Only relevant when enabled == false. Note that
   // old clients (<M45) won't set this, even when enabled is false.
   optional int32 disable_reasons = 10;
 }