DevTools: Get rid of Element.prototype.enableStyleClass

Source/devtools/front_end/ThreadsToolbar.js

 /*
  * Copyright (C) 2014 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 93 matching lines @@
         var selectedOption = this._comboBox.selectedOption();
         if (!selectedOption)
             return;
 
         WebInspector.workerManager.setSelectedThreadId(parseInt(selectedOption.value, 10));
     },
 
     _alterVisibility: function()
     {
         var hidden = this._comboBox.size() === 1;
-        this.element.enableStyleClass("hidden", hidden);
+        this.element.classList.toggle("hidden", hidden);
     }
 
 }