showContent(layer, withoutOverlay) {
    this._beforeShowOrEmptyLayer();
    this._contentPromise = this._contentOnlyPromise = this._contentContainer.show(layer);
    this._contentOnlyPromise.nodeify(this._afterShowOrEmptyLayer.bind(this));

    if (withoutOverlay && this._overlay.getCurrentLayer() != null) {
      this._contentPromise = Promise.all([this._contentOnlyPromise, this.destroyOverlay()]);
    }

    return this._contentPromise;
  }
Shows a content layer. Returns a promise that resolves when the content has been fully swapped.
@param {cc.Layer} layer
@param [withoutOverlay=false] whether to clear overlay
@returns {Promise}