DevTools: fix private field usage violation discovered after the compiler roll.

third_party/WebKit/Source/devtools/front_end/accessibility/AXTreePane.js

 // Copyright 2016 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.
 /**
  * @unrestricted
  */
 Accessibility.AXTreePane = class extends Accessibility.AccessibilitySubPane {
   constructor() {
     super(Common.UIString('Accessibility Tree'));
 
@@ skipping 339 matching lines @@
     this._expandSiblingsButton = new Accessibility.ExpandSiblingsButton(this, numSiblings);
     this._partiallyExpanded = false;
   }
 
   /**
    * @override
    */
   onattach() {
     super.onattach();
     if (this._treePane.isExpanded(this._axNode.backendDOMNodeId()))
-      this._listItemNode.classList.add('siblings-expanded');
+      this.listItemElement.classList.add('siblings-expanded');
     if (this._axNode.numChildren() > 1)
-      this._listItemNode.insertBefore(this._expandSiblingsButton.element, this._inspectNodeButton.element);
+      this.listItemElement.insertBefore(this._expandSiblingsButton.element, this._inspectNodeButton.element);
   }
 
   /**
    * @param {boolean} altKey
    * @return {boolean}
    * @override
    */
   descendOrExpand(altKey) {
     if (!this.expanded || !this._partiallyExpanded)
       return super.descendOrExpand(altKey);
 
     this.expandSiblings();
     if (altKey)
       this.expandRecursively();
     return true;
   }
 
   /**
    * @override
    */
   expand() {
     super.expand();
     this._partiallyExpanded = true;
   }
 
   expandSiblings() {
-    this._listItemNode.classList.add('siblings-expanded');
+    this.listItemElement.classList.add('siblings-expanded');
     this.appendSiblings();
     this.expanded = true;
     this._partiallyExpanded = false;
     this._treePane.setExpanded(this._axNode.backendDOMNodeId(), true);
   }
 
   appendSiblings() {
     var inspectedAXNode = this._inspectedNodeTreeElement.axNode();
     var nextIndex = 0;
     var foundInspectedNode = false;
     for (var sibling of this._axNode.children()) {
       var siblingTreeElement = null;
       if (sibling === inspectedAXNode) {
         foundInspectedNode = true;
         continue;
       }
       siblingTreeElement = new Accessibility.AXNodeTreeElement(sibling, this._treePane);
       if (foundInspectedNode)
         this.appendChild(siblingTreeElement);
       else
         this.insertChild(siblingTreeElement, nextIndex++);
     }
   }
 };