DevTools: migrate sidebar pane's titleElement to use Toolbar.

Source/devtools/front_end/elements/ElementsSidebarView.js

 // Copyright (c) 2015 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.SidebarPane}
  * @param {string} title
  * @param {!WebInspector.Widget} widget
  */
 WebInspector.ElementsSidebarViewWrapperPane = function(title, widget)
 {
     WebInspector.SidebarPane.call(this, title);
-    widget.show(this.bodyElement);
+    widget.show(this.element);
 }
 
 WebInspector.ElementsSidebarViewWrapperPane.prototype = {
     __proto__: WebInspector.SidebarPane.prototype
 }