static defaultGetContentId(initData) {
    const uriString = shaka.util.StringUtils.fromBytesAutoDetect(initData);

    // The domain of that URI is the content ID according to Apple's FPS
    // sample.
    const uri = new goog.Uri(uriString);
    return uri.getDomain();
  }
Using the default method, extract a content ID from the init data.  This is
based on the FairPlay example documentation.

@param {!BufferSource} initData
@return {string}
@export