Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/app_list_specifics.proto

 // Copyright 2013 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 the app list (aka app launcher).
 
 // Update proto_{value,enum}_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 app list objects.
 message AppListSpecifics {
   // Unique identifier for the item:
   // * TYPE_FOLDER: Folder id (generated)
   // * TYPE_APP: App Id
   // * TYPE_URL: Url
   optional string item_id = 1;
@@ skipping 20 matching lines @@
   // Marked OBSOLETE because this is unused for the app list.
   // Which page this item will appear on in the app list.
   optional string OBSOLETE_page_ordinal = 5 [deprecated = true];
 
   // Where on a page this item will appear.
   optional string item_ordinal = 6;
 
   // Where on a shelf this item will appear.
   optional string item_pin_ordinal = 7;
 }