[css-typed-om] use undefined union value for StylePropertyMapReadOnly.get()

whatwg/webidl#60 has been fixed for a long time.

Author: saschanaz

css-typed-om/Overview.bs

@@ -314,9 +314,7 @@ The {{StylePropertyMap}} {#the-stylepropertymap}
 [Exposed=(Window, Worker, PaintWorklet, LayoutWorklet)]
 interface StylePropertyMapReadOnly {
     iterable<USVString, sequence<CSSStyleValue>>;
-    any get(USVString property);
-    /* 'any' means (undefined or CSSStyleValue) here,
-       see https://github.com/heycam/webidl/issues/60 */
+    (undefined or CSSStyleValue) get(USVString property);
     sequence<CSSStyleValue> getAll(USVString property);
     boolean has(USVString property);
     readonly attribute unsigned long size;