DevTools: do not use arrow functions unless they are in stable.

Source/devtools/front_end/network/NetworkPanel.js

 /*
  * Copyright (C) 2007, 2008 Apple Inc.  All rights reserved.
  * Copyright (C) 2008, 2009 Anthony Ricaud <rik@webkit.org>
  * Copyright (C) 2011 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
@@ skipping 186 matching lines @@
     /**
      * @param {?WebInspector.FilmStripModel} filmStripModel
      */
     _filmStripAvailable: function(filmStripModel)
     {
         if (!filmStripModel)
             return;
         var calculator = this._networkLogView.timeCalculator();
         this._filmStripView.setModel(filmStripModel, calculator.minimumBoundary() * 1000, calculator.boundarySpan() * 1000);
         this._networkOverview.setFilmStripModel(filmStripModel);
-        var timestamps = filmStripModel.frames().map((frame) => frame.timestamp / 1000);
+        var timestamps = filmStripModel.frames().map(mapTimestamp);
+
+        /**
+         * @param {!WebInspector.FilmStripModel.Frame} frame
+         * @return {number}
+         */
+        function mapTimestamp(frame)
+        {
+            return frame.timestamp / 1000;
+        }
+
         this._networkLogView.addFilmStripFrames(timestamps);
     },
 
     /**
      * @param {!Event} event
      */
     _onPreserveLogCheckboxChanged: function(event)
     {
         this._networkLogView.setPreserveLog(this._preserveLogCheckbox.checked());
     },
@@ skipping 503 matching lines @@
     {
         if (!this._target)
             return;
 
         this._target.tracingManager.stop();
         this._target = null;
         this._callback = callback;
         this._filmStripView.setStatusText(WebInspector.UIString("Fetching frames..."));
     }
 }