Implement CSSTransitionDiscrete which enables transitions on discrete…

… properties.

This feature enables transitioning discrete properties.

This is based on the resolved CSSWG issue:
- Transition discrete properties: w3c/csswg-drafts#4441

You can test display transitions with this CL on:
https://output.jsbin.com/buquher/quiet

Original patch: http://crrev.com/c/4182089

Bug: 1399631, 1413556
Change-Id: Icf5364b970233186794fa9faaa016e0e6542ec2b