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

third_party/WebKit/Source/devtools/front_end/sources/WatchExpressionsSidebarPane.js

 /*
  * Copyright (C) IBM Corp. 2009  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 232 matching lines @@
     this._element = createElementWithClass("div", "watch-expression monospace");
     this._editing = false;
     this._linkifier = linkifier;
 
     this._createWatchExpression(null);
     this.update();
 }
 
 WebInspector.WatchExpression._watchObjectGroupId = "watch-group";
 
+/** @enum {symbol} */
 WebInspector.WatchExpression.Events = {
-    ExpressionUpdated: "ExpressionUpdated"
+    ExpressionUpdated: Symbol("ExpressionUpdated")
 }
 
 WebInspector.WatchExpression.prototype = {
 
     /**
      * @return {!Element}
      */
     element: function()
     {
         return this._element;
@@ skipping 184 matching lines @@
             contextMenu.appendApplicableItems(this._result);
     },
 
     _copyValueButtonClicked: function()
     {
         InspectorFrontendHost.copyText(this._valueElement.textContent);
     },
 
     __proto__: WebInspector.Object.prototype
 }