constructor(key) {
    /**
     * The key used to request the high-entropy value from the browser.
     * @const {string}
     * @private
     */
    this.key_ = key;

    /**
     * The value represented by this HighEntropyValue instance. If it hasn't
     * been successfully requested yet, its value will be undefined.
     * @type {VALUE_TYPE|undefined}
     * @protected
     */
    this.value_ = undefined;

    /**
     * The high-entropy value request. If it hasn't been requested yet, this
     * value will be undefined.
     * @type {!Promise<VALUE_TYPE>|undefined}
     * @private
     */
    this.promise_ = undefined;

    this.pending_ = false;
  }
Constructs a new HighEntropyValue instance.
@param {string} key The name of the high-entropy value, used when
requesting it from the browser.