Populate FileDescriptorProto for MessageFactory's DescriptorPool.

def _MakeFileDescriptorProto(proto_file_name, full_name, field_items):
  """Populate FileDescriptorProto for MessageFactory's DescriptorPool."""
  package, name = full_name.rsplit('.', 1)
  file_proto = descriptor_pb2.FileDescriptorProto()
  file_proto.name = os.path.join(package.replace('.', '/'), proto_file_name)
  file_proto.package = package
  desc_proto = file_proto.message_type.add()
  desc_proto.name = name
  for f_number, (f_name, f_type) in enumerate(field_items, 1):
    field_proto = desc_proto.field.add()
    field_proto.name = f_name
    field_proto.number = f_number
    field_proto.label = descriptor_pb2.FieldDescriptorProto.LABEL_OPTIONAL
    field_proto.type = f_type
  return file_proto