Convert map field value for a message map field.

    Args:
      value: A JSON object to convert the map field value.
      message: A protocol message to record the converted data.
      field: The descriptor of the map field to be converted.

    Raises:
      ParseError: In case of convert problems.

def _ConvertMapFieldValue(self, value, message, field):
    """Convert map field value for a message map field.

    Args:
      value: A JSON object to convert the map field value.
      message: A protocol message to record the converted data.
      field: The descriptor of the map field to be converted.

    Raises:
      ParseError: In case of convert problems.
    """
    if not isinstance(value, dict):
      raise ParseError(
          'Map field {0} must be in a dict which is {1}.'.format(
              field.name, value))
    key_field = field.message_type.fields_by_name['key']
    value_field = field.message_type.fields_by_name['value']
    for key in value:
      key_value = _ConvertScalarFieldValue(key, key_field, True)
      if value_field.cpp_type == descriptor.FieldDescriptor.CPPTYPE_MESSAGE:
        self.ConvertMessage(value[key], getattr(
            message, field.name)[key_value])
      else:
        getattr(message, field.name)[key_value] = _ConvertScalarFieldValue(
            value[key], value_field)