Bug 1576892: Turn on dom.animations-api.implicit-keyframes.enabled for css-text tests. r=birtles

Depends on D44100

Differential Revision: https://phabricator.services.mozilla.com/D44101

--HG--
extra : moz-landing-system : lando

testing/web-platform/meta/css/css-text/animations/__dir__.ini

@@ -1 +1,2 @@
+prefs: [dom.animations-api.implicit-keyframes.enabled:true]
 leak-threshold: [default:51200]