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

Source/devtools/front_end/timeline/TimelineModelImpl.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
  * @extends {WebInspector.TimelineModel}
  * @param {!WebInspector.TimelineManager} timelineManager
  */
 WebInspector.TimelineModelImpl = function(timelineManager)
@@ skipping 281 matching lines @@
  * @implements {WebInspector.TimelineModel.Record}
  * @param {!WebInspector.TimelineModelImpl} model
  * @param {!TimelineAgent.TimelineEvent} timelineEvent
  * @param {?WebInspector.TimelineModel.Record} parentRecord
  */
 WebInspector.TimelineModel.RecordImpl = function(model, timelineEvent, parentRecord)
 {
     this._model = model;
     var bindings = this._model._bindings;
     this._record = timelineEvent;
+    this._thread = this._record.thread || WebInspector.TimelineModel.MainThreadName;
     this._children = [];
     if (parentRecord) {
         this.parent = parentRecord;
         parentRecord.children().push(this);
     }
 
     this._selfTime = this.endTime() - this.startTime();
 
     var recordTypes = WebInspector.TimelineModel.RecordType;
     switch (timelineEvent.type) {
@@ skipping 103 matching lines @@
 
     /**
      * @return {number}
      */
     startTime: function()
     {
         return this._record.startTime;
     },
 
     /**
-     * @return {string|undefined}
+     * @return {string}
      */
     thread: function()
     {
-        return this._record.thread;
+        return this._thread;
     },
 
     /**
      * @return {number}
      */
     endTime: function()
     {
         return this._endTime || this._record.endTime || this._record.startTime;
     },
 
@@ skipping 260 matching lines @@
                 break;
             length += itemLength;
             data.push(item);
             ++this._recordIndex;
         }
         if (this._recordIndex === this._payloads.length)
             data.push(data.pop() + "]");
         stream.write(data.join(separator), this._writeNextChunk.bind(this));
     }
 }