DevTools: avoid layout thrashing on Layers panel

third_party/WebKit/Source/devtools/front_end/timeline/LayerTreeOutline.js

 /*
  * Copyright (C) 2013 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 109 matching lines @@
             if (!layer.drawsContent() && !showInternalLayers)
                 return;
             if (seenLayers.get(layer))
                 console.assert(false, "Duplicate layer: " + layer.id());
             seenLayers.set(layer, true);
             var node = layer[WebInspector.LayerTreeElement._symbol];
             var parentLayer = layer.parent();
             // Skip till nearest visible ancestor.
             while (parentLayer && parentLayer !== root && !parentLayer.drawsContent() && !showInternalLayers)
                 parentLayer = parentLayer.parent();
-            var parent = layer === root ? this._treeOutline : parentLayer[WebInspector.LayerTreeElement._symbol];
+            var parent = layer === root ? this._treeOutline.rootElement() : parentLayer[WebInspector.LayerTreeElement._symbol];
             if (!parent) {
                 console.assert(false, "Parent is not in the tree");
                 return;
             }
             if (!node) {
                 node = new WebInspector.LayerTreeElement(this, layer);
                 parent.appendChild(node);
                 // Expand all new non-content layers to expose content layers better.
                 if (!layer.drawsContent())
                     node.expand();
@@ skipping 111 matching lines @@
     /**
      * @param {boolean} hovered
      */
     setHovered: function(hovered)
     {
         this.listItemElement.classList.toggle("hovered", hovered);
     },
 
     __proto__: TreeElement.prototype
 }