DevTools: migrate treeoutline from hasChildren to is/setExpandable().

Source/devtools/front_end/elements/ElementsTreeElement.js

 /*
  * Copyright (C) 2007, 2008 Apple Inc.  All rights reserved.
  * Copyright (C) 2008 Matt Lilek <webkit@mattlilek.com>
  * Copyright (C) 2009 Joseph Pecoraro
  *
  * 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 407 matching lines @@
      * @return {boolean}
      */
     ondblclick: function(event)
     {
         if (this._editing || this._elementCloseTag)
             return false;
 
         if (this._startEditingTarget(/** @type {!Element} */(event.target)))
             return false;
 
-        if (this.hasChildren && !this.expanded)
+        if (this.isExpandable() && !this.expanded)
             this.expand();
         return false;
     },
 
     /**
      * @return {boolean}
      */
     hasEditableNode: function()
     {
         return !this._node.isShadowRoot() && !this._node.ancestorClosedShadowRoot();
@@ skipping 1165 matching lines @@
 
             if (object)
                 object.callFunction(scrollIntoView);
         }
 
         this._node.resolveToObject("", scrollIntoViewCallback);
     },
 
     __proto__: TreeElement.prototype
 }