constructor() {
    super();
    /** @type {LH.Gatherer.ProtocolSession|undefined} */
    this._session = undefined;
    /** @type {Map<string, Promise<LH.Artifacts.CSSStyleSheetInfo|null>>} */
    this._sheetPromises = new Map();
    this._onStylesheetAdded = this._onStylesheetAdded.bind(this);
  }
@fileoverview Collects all stylesheets and their contents.