static makeSimpleManifestText(lines, duration, startTime) {
    let periodAttr = '';
    let mpdAttr = 'type="dynamic" availabilityStartTime="1970-01-01T00:00:00Z"';
    if (duration) {
      periodAttr = 'duration="PT' + duration + 'S"';
      mpdAttr = 'type="static"';
    }
    if (startTime) {
      periodAttr += ' start="PT' + startTime + 'S"';
    }

    const start = [
      '<MPD ' + mpdAttr + '>',
      '  <Period ' + periodAttr + '>',
      '    <AdaptationSet mimeType="video/mp4">',
      '      <Representation bandwidth="500">',
      '        <BaseURL>http://example.com</BaseURL>',
    ];
    const end = [
      '      </Representation>',
      '    </AdaptationSet>',
      '  </Period>',
      '</MPD>',
    ];
    return start.concat(lines, end).join('\n');
  }
Makes a simple manifest with the given representation contents.

@param {!Array<string>} lines
@param {number=} duration
@param {number=} startTime
@return {string}