Bug 1574379 [wpt PR 18480] - Update interfaces/event-timing.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/event-timing.idl (#18480)

Source: https://github.com/tidoust/reffy-reports/blob/48ca519/whatwg/idl/event-timing.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/572612386
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wpt-commits: 01fd83a5cff33cb204cece9736cd7d5bb2c02b53
wpt-pr: 18480

testing/web-platform/tests/interfaces/event-timing.idl

@@ -8,6 +8,7 @@ interface PerformanceEventTiming : PerformanceEntry {
     readonly attribute DOMHighResTimeStamp processingStart;
     readonly attribute DOMHighResTimeStamp processingEnd;
     readonly attribute boolean cancelable;
+    [Default] object toJSON();
 };
 
 [Exposed=Window]