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

Source/devtools/front_end/timeline/TimelineModel.js

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 481 matching lines @@
 
     /**
      * @return {!Array.<!WebInspector.TimelineModel.Record>}
      */
     records: function()
     {
         return this._records;
     },
 
     /**
+     * @return {?WebInspector.Target}
+     */
+    target: function()
+    {
+        // FIXME: Consider returning null for loaded traces.
+        return this._tracingManager.target();
+    },
+
+    /**
      * @param {!Array.<!WebInspector.TracingManager.EventPayload>} events
      */
     setEventsForTest: function(events)
     {
         this._startCollectingTraceEvents(false);
         this._tracingModel.addEvents(events);
         this._onTracingComplete();
     },
 
     _startCpuProfilingOnAllTargets: function()
@@ skipping 1175 matching lines @@
     _initializePerFrameState: function()
     {
         /** @type {!Array.<!WebInspector.InvalidationTrackingEvent>} */
         this._invalidationEvents = [];
         this._lastStyleRecalcEventIndex = 0;
         this._lastLayoutEventIndex = 0;
         this._lastPaintWithLayer = undefined;
         this._didPaint = false;
     }
 }