constructor() {
    super();
    /**
     * @private {!Object<string, {value: string, expiration: number}>} The mock
     *     cookie storage hash map.
     */
    this.mockCookieStorage_ = {};
    /**
     * @private {?PropertyReplacer}
     */
    this.replacer_ = null;
  }
Fake cookie storage client class. This makes it safe to isolate cookies
between tests and doesn't require manual cleaning each time. It also makes it
easy to test cookie expiration with MockClock.