static parseFromProtection(elements, keySystemsBySystemId) {
    const ContentProtection = shaka.mss.ContentProtection;
    const TXml = shaka.util.TXml;

    /** @type {!Array<!shaka.extern.xml.Node>} */
    let protectionHeader = [];
    for (const element of elements) {
      protectionHeader = protectionHeader.concat(
          TXml.findChildren(element, 'ProtectionHeader'));
    }
    if (!protectionHeader.length) {
      return [];
    }
    return ContentProtection.convertElements_(
        protectionHeader, keySystemsBySystemId);
  }
Parses info from the Protection elements.

@param {!Array<!shaka.extern.xml.Node>} elements
@param {!Object<string, string>} keySystemsBySystemId
@return {!Array<shaka.extern.DrmInfo>}