Bug 1726182 - Add a use counter for content-visibility. r=dholbert

Differential Revision: https://phabricator.services.mozilla.com/D122873

servo/components/style/properties/counted_unknown_properties.py

@@ -121,4 +121,5 @@ COUNTED_UNKNOWN_PROPERTIES = [
     "-webkit-columns",
     "-webkit-column-rule-color",
     "-webkit-shape-margin",
+    "content-visibility",
 ]