DevTools: Make target on TimelineRecord nullable

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 374 matching lines @@
 
     /**
      * @return {?WebInspector.TimelineModel.Record}
      */
     initiator: function()
     {
         return this._initiator;
     },
 
     /**
-     * @return {!WebInspector.Target}
+     * @return {?WebInspector.Target}
      */
     target: function()
     {
         return this._model._target;
     },
 
     /**
      * @return {number}
      */
     selfTime: function()
@@ skipping 296 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));
     }
 }