Update interfaces/performance-timeline.idl (#14276)

Source: https://github.com/tidoust/reffy-reports/blob/6f59720/whatwg/idl/performance-timeline.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/460633569
diff --git a/interfaces/performance-timeline.idl b/interfaces/performance-timeline.idl
index 2ec8247..0cd2830 100644
--- a/interfaces/performance-timeline.idl
+++ b/interfaces/performance-timeline.idl
@@ -26,6 +26,7 @@
   void observe(PerformanceObserverInit options);
   void disconnect();
   PerformanceEntryList takeRecords();
+  static readonly attribute FrozenArray<DOMString> supportedEntryTypes;
 };
 
 dictionary PerformanceObserverInit {