DevTools: Support multiple target in TimelineModelImpl

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

 var initialize_Timeline = function() {
 
 // Scrub values when printing out these properties in the record or data field.
 InspectorTest.timelinePropertyFormatters = {
     children: "formatAsTypeName",
     endTime: "formatAsTypeName",
     requestId: "formatAsTypeName",
     startTime: "formatAsTypeName",
     stackTrace: "formatAsTypeName",
     url: "formatAsURL",
@@ skipping 25 matching lines @@
 InspectorTest.timelineModel = function()
 {
     return WebInspector.panels.timeline._model;
 }
 
 InspectorTest.startTimeline = function(callback)
 {
     InspectorTest._timelineRecords = [];
     WebInspector.inspectorView.panel("timeline").toggleTimelineButton.toggled = true;
     WebInspector.inspectorView.panel("timeline")._model._collectionEnabled = true;
+    WebInspector.inspectorView.panel("timeline")._model._currentTarget = WebInspector.targetManager.mainTarget();
+
     TimelineAgent.start(5, false, undefined, true, false, callback);
     function addRecord(record)
     {
         InspectorTest._timelineRecords.push(record);
         for (var i = 0; record.children && i < record.children.length; ++i)
             addRecord(record.children[i]);
     }
     InspectorTest._addTimelineEvent = function(event)
     {
         addRecord(event.data);
@@ skipping 299 matching lines @@
         return this._fileSize;
     },
 
     fileName: function()
     {
         return "fakeFile";
     }
 };
 
 };