Work around IE 11 bug in text region positioning
Work around IE 11 bug in text region positioning

While developing screenshot-based layout tests, it was discovered that
IE 11 did not position VTT/TTML regions in the same way as other
browsers.  This turned out to be caused by a bug in IE's flexbox
implementation in which flex items that are center-positioned can
overflow the flex parent.

This uses the work-around recommended here:
https://github.com/philipwalton/flexbugs/tree/6e720da8#flexbug-2

Closes #2584

Change-Id: I22b9790b8f5798ba757a111d2c502dd5a3415441

tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player