DevTools: Use JS symbols instead of strings for eligible events.

third_party/WebKit/Source/devtools/front_end/timeline_model/LayerTreeModel.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 48 matching lines @@
   */
 WebInspector.LayerTreeModel = function(target)
 {
     WebInspector.SDKModel.call(this, WebInspector.LayerTreeModel, target);
     target.registerLayerTreeDispatcher(new WebInspector.LayerTreeDispatcher(this));
     WebInspector.targetManager.addEventListener(WebInspector.TargetManager.Events.MainFrameNavigated, this._onMainFrameNavigated, this);
     /** @type {?WebInspector.LayerTreeBase} */
     this._layerTree = null;
 }
 
+/** @enum {symbol} */
 WebInspector.LayerTreeModel.Events = {
-    LayerTreeChanged: "LayerTreeChanged",
-    LayerPainted: "LayerPainted",
+    LayerTreeChanged: Symbol("LayerTreeChanged"),
+    LayerPainted: Symbol("LayerPainted"),
 }
 
 WebInspector.LayerTreeModel.ScrollRectType = {
     NonFastScrollable: {name: "NonFastScrollable", description: "Non fast scrollable"},
     TouchEventHandler: {name: "TouchEventHandler", description: "Touch event handler"},
     WheelEventHandler: {name: "WheelEventHandler", description: "Wheel event handler"},
     RepaintsOnScroll: {name: "RepaintsOnScroll", description: "Repaints on scroll"}
 }
 
 WebInspector.LayerTreeModel.prototype = {
@@ skipping 1218 matching lines @@
 WebInspector.LayerTreeModel.fromTarget = function(target)
 {
     if (!target.hasDOMCapability())
         return null;
 
     var model = /** @type {?WebInspector.LayerTreeModel} */ (target.model(WebInspector.LayerTreeModel));
     if (!model)
         model = new WebInspector.LayerTreeModel(target);
     return model;
 }