First version of PerformanceObserver

Source/core/timing/PerformanceEntry.idl

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 17 matching lines @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 // https://w3c.github.io/performance-timeline/#the-performanceentry-interface
 
 [
     // TODO(philipj): Exposed=(Window,Worker)
     GarbageCollected,
 ] interface PerformanceEntry {
     readonly attribute DOMString name;
-    readonly attribute DOMString entryType;
+    readonly attribute PerformanceEntryType entryType;
     readonly attribute DOMHighResTimeStamp startTime;
     readonly attribute DOMHighResTimeStamp duration;
     serializer = {attribute};
 };