Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/app_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 apps.
 
 // 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;
 
 import "extension_specifics.proto";
 
 // Settings related to push notifications for apps.
 message AppNotificationSettings {
   // DEPRECATED: Use oauth_client_id below.
   // Whether or not the user has setup notifications at least once.
   // The value for this field will start out false and will be set
@@ skipping 62 matching lines @@
   optional string bookmark_app_description = 7;
 
   // This is the color to use when generating bookmark app icons. The string is
   // in #rrggbb or #rgb syntax, e.g. #d8d8d8.
   optional string bookmark_app_icon_color = 8;
 
   // This is information about linked icons (that is, icons that are downloaded
   // from outside the app's bundle of files.
   repeated LinkedAppIconInfo linked_app_icons = 9;
 }