Reading list: add first read PB entries

components/sync/protocol/reading_list_specifics.proto

 // Copyright 2016 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 reading list items.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package sync_pb;
 
 // Sync Reading list entry. This proto contains the fields synced for a reading
 // list entry. It must be kept synced with the reading_list.ReadingListLocal
 // protobuf.
 message ReadingListSpecifics {
   optional string entry_id = 1;
-
   optional string title = 2;
-
   optional string url = 3;
-
   optional int64 creation_time_us = 4;
-
   optional int64 update_time_us = 5;
+  optional int64 first_read_time_us = 7;
 
   enum ReadingListEntryStatus {
     UNREAD = 0;
     READ = 1;
   }
 
   optional ReadingListEntryStatus status = 6;
 }