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

sync/util/time.cc

 // 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.
 
 #include "sync/util/time.h"
 
 #include "base/i18n/time_formatting.h"
 #include "base/strings/utf_string_conversions.h"
 
 namespace syncer {
 
 int64 TimeToProtoTime(const base::Time& t) {
-  return (t - base::Time::UnixEpoch()).InMilliseconds();
+  return t.is_null() ? 0 : (t - base::Time::UnixEpoch()).InMilliseconds();
 }
 
 base::Time ProtoTimeToTime(int64 proto_t) {
   return base::Time::UnixEpoch() + base::TimeDelta::FromMilliseconds(proto_t);
 }
 
 std::string GetTimeDebugString(const base::Time& t) {
   return base::UTF16ToUTF8(base::TimeFormatFriendlyDateAndTime(t));
 }
 
 }  // namespace syncer