Bug 1574379 [wpt PR 18480] - Update wpt metadata, a=testonly

wpt-pr: 18480
wpt-type: metadata

testing/web-platform/meta/event-timing/idlharness.any.js.ini

@@ -50,6 +50,9 @@
   [EventCounts interface object name]
     expected: FAIL
 
+  [PerformanceEventTiming interface: operation toJSON()]
+    expected: FAIL
+
 
 [idlharness.any.worker.html]
   [Performance interface: performance must inherit property "eventCounts" with the proper type]