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)

*   component.
 * @property {PDFFindController} [findController] - The find controller
 *   component.
 * @property {PDFScriptingManager} [scriptingManager] - The scripting manager
 *   component.
 * @property {PDFRenderingQueue} [renderingQueue] - The rendering queue object.
 * @property {boolean} [removePageBorders] - Removes the border shadow around
 *   the pages. The default value is `false`.
 * @property {number} [textLayerMode] - Controls if the text layer used for
 *   selection and searching is created, and if the improved text selection
 *   behaviour is enabled. The constants from {TextLayerMode} should be used.
 *   The default value is `TextLayerMode.ENABLE`.
 * @property {number} [annotationMode] - Controls if the annotation layer is
 *   created, and if interactive form elements or `AnnotationStorage`-data are
 *   being rendered. The constants from {@link AnnotationMode} should be used;
 *   see also {@link RenderParameters} and {@link GetOperatorListParameters}.
 *   The default value is `AnnotationMode.ENABLE_FORMS`.
 * @property {string} [imageResourcesPath] - Path for image resources, mainly
 *   mainly for annotation icons. Include trailing slash.
 * @property {boolean} [enablePrintAutoRotate] - Enables automatic rotation of
 *   landscape pages upon printing. The default is `false`.
 * @property {string} renderer - 'canvas' or 'svg'. The default is 'canvas'.
 * @property {boolean} [useOnlyCssZoom] - Enables CSS only zooming. The default
 *   value is `false`.
 * @property {number} [maxCanvasPixels] - The maximum supported canvas size in
 *   total pixels, i.e. width * height. Use -1 for no limit. The default value
 *   is 4096 * 4096 (16 mega-pixels).
 * @property {IL10n} l10n - Localization service.
 */
class PDFPageViewBuffer {