Fix devtools timeline aggregated test

This test includes a JSSample event that has no stackTrace property
which is not valid. The frontend assumes this property always exists
even if it is an empty array.

Drive-by fix the unused `model` var and change event to a const.

Bug: 1127827
Change-Id: Ia33e0103ba85b22e7ab18c7d7074935d65130c2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2454083Reviewed-by: Paul Lewis <aerotwist@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814648}

third_party/blink/web_tests/http/tests/devtools/tracing/timeline-js/timeline-open-function-call.js

@@ -55,12 +55,12 @@
       'pid': 17851,
       'tid': 23,
       'ts': 142000,
-      'args': {}
+      'args': {'data': {'stackTrace': []}}
     }
   ];
 
-  var model = PerformanceTestRunner.createPerformanceModelWithEvents(rawTraceEvents);
-  var event = PerformanceTestRunner.mainTrackEvents().find(
+  PerformanceTestRunner.createPerformanceModelWithEvents(rawTraceEvents);
+  const event = PerformanceTestRunner.mainTrackEvents().find(
       e => e.name === TimelineModel.TimelineModel.RecordType.FunctionCall);
   TestRunner.addResult(`${event.startTime} ${event.endTime}`);
   TestRunner.completeTest();