Timeline: fix expansion arrow appearing on rows with no visible children

Source/devtools/front_end/TimelinePresentationModel.js

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  * Copyright (C) 2012 Intel 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.
@@ skipping 328 matching lines @@
      * @return {!Array.<!WebInspector.TimelinePresentationModel.Record>}
      */
     presentationChildren: function()
     {
         return this._presentationChildren;
     },
 
     /**
      * @return {boolean}
      */
-    hasPresentationChildren: function()
-    {
-        return !!this._presentationChildren.length;
-    },
-
-    /**
-     * @return {boolean}
-     */
     coalesced: function()
     {
         return this._coalesced;
     },
 
     /**
      * @return {boolean}
      */
     collapsed: function()
     {
@@ skipping 74 matching lines @@
     },
 
     /**
      * @param {!WebInspector.TimelineRecordGraphRow} graphRow
      */
     setGraphRow: function(graphRow)
     {
         this._graphRow = graphRow;
     }
 }