Method whenFirstPreviewed

src/service/ampdoc-impl.js:673–677

* Returns a Promise that only ever resolved when the current * AMP document first reaches the `PREVIEW` visibility state. * @return {!Promise}

()

   * @return {!Promise}
   */
  whenFirstPreviewed() {
    return this.signals_
      .whenSignal(AmpDocSignals_Enum.FIRST_PREVIEWED)
      .then(() => undefined);
  }

  /**
   * Returns a Promise that only ever resolved when the current