Stores <config_option> in the writeback.

def Persist(self, config_option):
    """Stores <config_option> in the writeback."""
    if not self.writeback:
      raise RuntimeError("Attempting to write a configuration without a "
                         "writeback location.")

    writeback_raw_value = dict(self.writeback.RawData()).get(config_option)
    raw_value = None

    for parser in [self.parser] + self.secondary_config_parsers:
      if parser == self.writeback:
        continue

      config_raw_data = dict(parser.RawData())
      raw_value = config_raw_data.get(config_option)
      if raw_value is None:
        continue
      break

    if writeback_raw_value == raw_value:
      return

    if raw_value is None:
      return

    self.SetRaw(config_option, raw_value)
    self.Write()