Timeline: do not imply event.thread.target is the main target

LayoutTests/http/tests/inspector/timeline-test.js

 var initialize_Timeline = function() {
 
 InspectorTest.preloadPanel("timeline");
 WebInspector.TempFile = InspectorTest.TempFileMock;
 
 // Scrub values when printing out these properties in the record or data field.
 InspectorTest.timelinePropertyFormatters = {
     children: "formatAsTypeName",
     endTime: "formatAsTypeName",
     requestId: "formatAsTypeName",
@@ skipping 86 matching lines @@
 InspectorTest.invokeAsyncWithTimeline = function(functionName, doneCallback)
 {
     InspectorTest.startTimeline(step1);
     function step1()
     {
         InspectorTest.invokePageFunctionAsync(functionName, step2);
     }
 
     function step2()
     {
-       InspectorTest.stopTimeline(doneCallback);
+        InspectorTest.stopTimeline(InspectorTest.safeWrap(doneCallback));
     }
 }
 
 InspectorTest.loadTimelineRecords = function(records)
 {
     var model = WebInspector.panels.timeline._model;
     model.reset();
     records.forEach(model._addRecord, model);
 }
 
@@ skipping 244 matching lines @@
         return this._fileSize;
     },
 
     fileName: function()
     {
         return "fakeFile";
     }
 };
 
 };