Merge "Specify --max_timestamp when calling brillo_update_payload." into nyc-dr1-dev

         "--target_image", target_file]

  if source_file is not None:

    cmd.extend(["--source_image", source_file])

  if OPTIONS.downgrade:

    max_timestamp = GetBuildProp("ro.build.date.utc", OPTIONS.source_info_dict)

  else:

    max_timestamp = metadata["post-timestamp"]

  cmd.extend(["--max_timestamp", max_timestamp])

  p1 = common.Run(cmd, stdout=subprocess.PIPE)

  p1.wait()

  assert p1.returncode == 0, "brillo_update_payload generate failed"