Bug 1659915 [wpt PR 25100] - Update interfaces/css-properties-values-api.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/css-properties-values-api.idl (#25100)

Source: https://github.com/tidoust/reffy-reports/blob/4cba804/ed/idl/css-properties-values-api.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/719138445
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wpt-commits: 4aa65f94b1effc5de489172b45a2973a8b82a686
wpt-pr: 25100

testing/web-platform/tests/interfaces/css-properties-values-api.idl

@@ -11,7 +11,7 @@ dictionary PropertyDefinition {
 };
 
 partial namespace CSS {
-  void registerProperty(PropertyDefinition definition);
+  undefined registerProperty(PropertyDefinition definition);
 };
 
 [Exposed=Window]