Bind the field descriptor to the owner once the target is defined.

def LateBind(self, target=None):
    """Bind the field descriptor to the owner once the target is defined."""
    self.type = target
    self._GetPrimitiveEncoder()

    # Now re-add the descriptor to the owner protobuf.
    self.late_bound = False
    self.owner.AddDescriptor(self)