Set Duration by seconds and nonas.

def _NormalizeDuration(self, seconds, nanos):
    """Set Duration by seconds and nonas."""
    # Force nanos to be negative if the duration is negative.
    if seconds < 0 and nanos > 0:
      seconds += 1
      nanos -= _NANOS_PER_SECOND
    self.seconds = seconds
    self.nanos = nanos