[sync] Add backup time in synced device info so that server can flag device

sync/protocol/device_info_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 nigori keys.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
@@ skipping 16 matching lines @@
   optional string client_name = 2;
 
   // The platform of the device.
   optional SyncEnums.DeviceType device_type = 3;
 
   // The UserAgent used when contacting the Chrome Sync server.
   optional string sync_user_agent = 4;
 
   // The Chrome instance's version.  Updated (if necessary) on every startup.
   optional string chrome_version = 5;
+
+  // Last time when pre-sync data on the device was saved. The device can be
+  // restored to state back to this time. In millisecond since UNIX epoch.
+  optional int64 backup_timestamp = 6;
 }