Fix memory leaks caused by PromiseRejectionEvents

LayoutTests/http/tests/dom/resources/promise-rejection-events.js

 'use strict';
 
 if (self.importScripts) {
   if ('ServiceWorkerGlobalScope' in self && self instanceof ServiceWorkerGlobalScope) {
     importScripts('../../serviceworker/resources/worker-testharness.js');
   } else {
     importScripts('../../resources/testharness.js');
   }
 }
 
@@ skipping 472 matching lines @@
         unhandledReasons.push(ev.reason);
       });
     }
   };
   var handled = function(ev) {
     if (ev.promise === p) {
       t.step(function() {
         assert_array_equals(unhandledPromises, [p]);
         assert_array_equals(unhandledReasons, [e]);
         assert_equals(ev.promise, p);
-        assert_equals(ev.reason, e);
+        assert_equals(ev.reason, undefined);
       });
     }
   };
   addEventListener('unhandledrejection', unhandled);
   addEventListener('rejectionhandled', handled);
   ensureCleanup(t, unhandled, handled);
 
   p = new Promise(function() {
     throw e;
   });
@@ skipping 155 matching lines @@
 function ensureCleanup(t, unhandled, handled) {
   t.add_cleanup(function() {
     if (unhandled)
       removeEventListener('unhandledrejection', unhandled);
     if (handled)
       removeEventListener('rejectionhandled', handled);
   });
 }
 
 done();