'video' component in 'media' example looks odd in Safari/Firefox

Do you want to request a feature or report a bug? This is a bug in the following example file: https://github.com/facebook/draft-js/blob/master/examples/draft-0-10-0/media/media.html

What is the current behavior? Steps to reproduce:

1. Open the 'media.html' file in Firefox or Safari.
2. Click 'Add Video'.
3. Enter 'media.mp4' in the text input.
4. Notice that the video component looks weird. See attached screenshot.

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem. You can use this jsfiddle to get started: https://jsfiddle.net/stopachka/m6z0xn4r/.

What is the expected behavior? It should not have a semi-transparent area or hang out of the Draft composer area.

Which versions of Draft.js, and which browser / OS are affected by this issue? Did this work in previous versions of Draft.js? Safari/Firefox with the latest Draft on the master branch.