Khmer and Burmese languages not rendered correctly

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**mapbox-gl-js version**: 0.53.1

**browser**: Firefox (Ubuntu 16.04)

### Steps to Trigger Behavior

1. Download zip containing examples together with pbf encoded fonts
[fontTest.zip](https://github.com/mapbox/mapbox-gl-js/files/2951741/fontTest.zip)
2. Host locally folder with fonts from zip on port 8080
3. Open provided html pages

### Expected Behavior
Label for Phnom Penh (Cambodia capital) is rendered correctly using proper fonts.

Label for Naypyidaw (Myanmar capital) is rendered correctly using proper fonts.

### Actual Behavior
Label for Phnom Penh is rendered incorrectly using google Noto Sans Khmer font (https://www.google.com/get/noto/#sans-khmr) on the right, and Khmer font (https://fonts.google.com/specimen/Khmer) on the left.

Label for Naypyidaw is rendered incorrectly using google Noto Sans Myanmar font (https://www.google.com/get/noto/#sans-mymr) on the left and Padauk font (https://fonts.google.com/?subset=myanmar) on the right.

Reproduce the problem using the linked fontTest.zip, its local HTML pages, and Firefox on Ubuntu 16.04; compare Khmer and Burmese labels with the expected images. Trace the text-rendering path to identify where these scripts diverge, and consider the work done when both labels render correctly with the supplied fonts and a regression test covers the behavior.

Written by the indexing model from the issue text.