constructor(resourceIds, generateBundles) {
    super(resourceIds, generateBundles);

    // A Set of DOM trees observed by the `MutationObserver`.
    this.roots = new Set();
    // requestAnimationFrame handler.
    this.pendingrAF = null;
    // list of elements pending for translation.
    this.pendingElements = new Set();
    this.windowElement = null;
    this.mutationObserver = null;

    this.observerConfig = {
      attributes: true,
      characterData: false,
      childList: true,
      subtree: true,
      attributeFilter: [L10NID_ATTR_NAME, L10NARGS_ATTR_NAME],
    };
  }
@param {Array<String>}    resourceIds     - List of resource IDs
@param {Function}         generateBundles - Function that returns a
                                            generator over FluentBundles
@returns {DOMLocalization}