Returns url, possibly with confirmation token.

def _get_drive_url(self, url, session):
    """Returns url, possibly with confirmation token."""
    response = session.get(url, stream=True)
    if response.status_code != 200:
      raise DownloadError(
          'Failed to get url %s. HTTP code: %d.' % (url, response.status_code))
    for k, v in response.cookies.items():
      if k.startswith('download_warning'):
        return url + '&confirm=' + v  # v is the confirm token
    # No token found, let's try with original URL:
    return url