Bug 1857729 - Run cssstyledeclaration-registered-custom-properties.html with layout.css.properties-and-values enabled. r=zrhoffman

This relies on @property.

Differential Revision: https://phabricator.services.mozilla.com/D190418

testing/web-platform/meta/css/cssom/cssstyledeclaration-registered-custom-properties.html.ini

@@ -1,3 +1,4 @@
 [cssstyledeclaration-registered-custom-properties.html]
+  prefs: [layout.css.properties-and-values.enabled:true]
   expected:
     if (os == "android") and fission: [OK, TIMEOUT]