Bug 1588792 - Enable the pref of individual transforms for will-change wpt in vendor-imports. r=hiro

Differential Revision: https://phabricator.services.mozilla.com/D49334

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

testing/web-platform/meta/css/vendor-imports/mozilla/mozilla-central-reftests/will-change/__dir__.ini

@@ -0,0 +1 @@
+prefs: [layout.css.individual-transform.enabled:true]