Returns this attribute's RDFValue class.

def GetRDFValueType(self):
    """Returns this attribute's RDFValue class."""
    result = self.attribute_type
    for field_name in self.field_names:
      # Support the new semantic protobufs.
      try:
        result = result.type_infos.get(field_name).type
      except AttributeError:
        raise AttributeError("Invalid attribute %s" % field_name)

    return result