DevTools: Unify main thread name across vanilla and tracing timeline modes.

Source/devtools/front_end/timeline/TracingTimelineModel.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @constructor
  * @param {!WebInspector.TracingModel} tracingModel
  * @param {!WebInspector.TimelineModel.Filter} recordFilter
  * @extends {WebInspector.TimelineModel}
  */
@@ skipping 525 matching lines @@
 
     /**
      * @return {number}
      */
     startTime: function()
     {
         return this._event.startTime;
     },
 
     /**
-     * @return {string|undefined}
+     * @return {string}
      */
     thread: function()
     {
-        return "CPU";
+        // FIXME: Should return the actual thread name.
+        return WebInspector.TimelineModel.MainThreadName;
     },
 
     /**
      * @return {number}
      */
     endTime: function()
     {
         return this._event.endTime || this._event.startTime;
     },
 
@@ skipping 91 matching lines @@
      * @param {!WebInspector.TracingTimelineModel.TraceEventRecord} child
      */
     _addChild: function(child)
     {
         this._children.push(child);
         child.parent = this;
     },
 
 
 }