def __init__(self, message_listener, message_descriptor):
super(RepeatedCompositeFieldContainer, self).__init__(message_listener)<EOL>self._message_descriptor = message_descriptor<EOL>
Note that we pass in a descriptor instead of the generated directly,
since at the time we construct a _RepeatedCompositeFieldContainer we
haven't yet necessarily initialized the type that will be contained in the
container.

Args:
  message_listener: A MessageListener implementation.
    The RepeatedCompositeFieldContainer will call this object's
    Modified() method when it is modified.
  message_descriptor: A Descriptor instance describing the protocol type
    that should be present in this container.  We'll use the
    _concrete_class field of this descriptor when the client calls add().