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

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

Index: third_party/WebKit/Source/devtools/front_end/accessibility/AXTreePane.js
diff --git a/third_party/WebKit/Source/devtools/front_end/accessibility/AXTreePane.js b/third_party/WebKit/Source/devtools/front_end/accessibility/AXTreePane.js
index 8dd1b5d35c8ffc60e0fc135f8462e3e63251c22e..f56fdaa3b88a092616a7451cb62892b9f3d2608a 100644
--- a/third_party/WebKit/Source/devtools/front_end/accessibility/AXTreePane.js
+++ b/third_party/WebKit/Source/devtools/front_end/accessibility/AXTreePane.js
@@ -357,9 +357,9 @@ Accessibility.AXNodeTreeParentElement = class extends Accessibility.AXNodeTreeEl
   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);
   }
 
   /**
@@ -386,7 +386,7 @@ Accessibility.AXNodeTreeParentElement = class extends Accessibility.AXNodeTreeEl
   }
 
   expandSiblings() {
-    this._listItemNode.classList.add('siblings-expanded');
+    this.listItemElement.classList.add('siblings-expanded');
     this.appendSiblings();
     this.expanded = true;
     this._partiallyExpanded = false;