style: Convert NS_STYLE_ANIMATION_PLAY_STATE_* to scoped enum StyleAn…

…imationPlayState.

This change removes directives NS_STYLE_ANIMATION_PLAY_STATE_* and replaces the
values with those from a scoped enum called StyleAnimationPlayState.

Differential Revision: https://phabricator.services.mozilla.com/D9382

tb120 authored and emilio committed Oct 28, 2018

ac873c7