Update interfaces/custom-state-pseudo-class.idl (#27231)

Source: https://github.com/w3c/webref/blob/8c43c9e/ed/idl/custom-state-pseudo-class.idl
Build: https://travis-ci.org/w3c/webref/builds/213523572
diff --git a/interfaces/custom-state-pseudo-class.idl b/interfaces/custom-state-pseudo-class.idl
index 9a0bd0c..0a71557 100644
--- a/interfaces/custom-state-pseudo-class.idl
+++ b/interfaces/custom-state-pseudo-class.idl
@@ -4,7 +4,13 @@
 // Source: Custom State Pseudo Class (https://wicg.github.io/custom-state-pseudo-class/)
 
 partial interface ElementInternals {
-  [SameObject, PutForwards=value] readonly attribute DOMTokenList states;
+  readonly attribute CustomStateSet states;
+};
+
+[Exposed=Window]
+interface CustomStateSet {
+  setlike<DOMString>;
+  undefined add(DOMString value);
 };
 
 [Exposed=Window]