static parseElements_(elements) {
    /** @type {!Array<shaka.dash.ContentProtection.Element>} */
    const out = [];

    for (const element of elements) {
      const parsed = shaka.dash.ContentProtection.parseElement_(element);
      if (parsed) {
        out.push(parsed);
      }
    }

    return out;
  }
Parses the given ContentProtection elements.  If there is an error, it
removes those elements.

@param {!Array<!shaka.extern.xml.Node>} elements
@return {!Array<shaka.dash.ContentProtection.Element>}
@private