Configuration:
- Web browser and its version: Firefox 67, Chrome 75
- Operating system and its version: Windows 10
- PDF.js version: 2.1.266
- Is a browser extension: no

Steps to reproduce the problem:
1. Open a PDF that needs the vertical centering. For example A4 paper displayed on the portrait display, or wide enough PDF page on the landscape orientation display.
2. Enter the presentiation mode.
3. The page is located on the top of the workspace, not in the middle.

What is the expected behavior? (add screenshot)

Expected vertical align in Adobe Reader XI:

![pdfjs-align-expected](https://user-images.githubusercontent.com/16005791/59553892-10bcf580-8f9c-11e9-9583-2a322e8b0ae3.png)

What went wrong? (add screenshot)

Current vertical align in PDF.js:

![pdfjs-align-bug](https://user-images.githubusercontent.com/16005791/59553866-d5222b80-8f9b-11e9-972a-98286da7b2d8.png)

const visiblePages = currentlyVisiblePages || this._getVisiblePages();
    const scrollAhead = this.#getScrollAhead(visiblePages);
    const preRenderExtra =
      this._spreadMode !== SpreadMode.NONE &&
      this._scrollMode !== ScrollMode.HORIZONTAL;
    const pageView = this.renderingQueue.getHighestPriority(
      visiblePages,
      this._pages,
      scrollAhead,
      preRenderExtra
    );
    this.#toggleLoadingIconSpinner(visiblePages.ids);
    if (pageView) {
      this.#ensurePdfPageLoaded(pageView).then(() => {
        this.renderingQueue.renderView(pageView);
      });
      return true;
    }
    return false;
  }
  /**
   * @param {HTMLDivElement} textLayerDiv
   * @param {number} pageIndex
   * @param {PageViewport} viewport
   * @param {boolean} enhanceTextSelection
   * @param {EventBus} eventBus
   * @param {TextHighlighter} highlighter
   * @returns {TextLayerBuilder}
   */
  createTextLayerBuilder(