DevTools: extract a component for layer viewer

third_party/WebKit/Source/devtools/front_end/layer_viewer/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 23 matching lines @@
  * @extends {WebInspector.Object}
  * @implements {WebInspector.LayerView}
  */
 WebInspector.LayerTreeOutline = function(layerViewHost)
 {
     WebInspector.Object.call(this);
     this._layerViewHost = layerViewHost;
     this._layerViewHost.registerView(this);
 
     this._treeOutline = new TreeOutlineInShadow();
-    this._treeOutline.element.classList.add("layer-tree");
+    this._treeOutline.element.classList.add("layer-tree", "overflow-auto");
     this._treeOutline.element.addEventListener("mousemove", this._onMouseMove.bind(this), false);
     this._treeOutline.element.addEventListener("mouseout", this._onMouseMove.bind(this), false);
     this._treeOutline.element.addEventListener("contextmenu", this._onContextMenu.bind(this), true);
 
     this._lastHoveredNode = null;
     this.element = this._treeOutline.element;
     this._layerViewHost.showInternalLayersSetting().addChangeListener(this._update, this);
 }
 
 WebInspector.LayerTreeOutline.prototype = {
@@ skipping 199 matching lines @@
     /**
      * @param {boolean} hovered
      */
     setHovered: function(hovered)
     {
         this.listItemElement.classList.toggle("hovered", hovered);
     },
 
     __proto__: TreeElement.prototype
 }