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)

/* Copyright 2014 Mozilla Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
import {
  AnnotationMode,
  createPromiseCapability,
  PixelsPerInch,
  version,
} from "pdfjs-lib";
import {
  DEFAULT_SCALE,
  DEFAULT_SCALE_DELTA,
  DEFAULT_SCALE_VALUE,
  getVisibleElements,
  isPortraitOrientation,
  isValidRotation,
  isValidScrollMode,
  isValidSpreadMode,
  MAX_AUTO_SCALE,