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

third_party/WebKit/Source/devtools/front_end/ui/treeoutline.js

 /*
  * Copyright (C) 2007 Apple 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 30 matching lines @@
     this._comparator = null;
 
     this.contentElement = this._rootElement._childrenListNode;
     this.contentElement.addEventListener("keydown", this._treeKeyDown.bind(this), true);
 
     this.setFocusable(!nonFocusable);
 
     this.element = this.contentElement;
 }
 
+/** @enum {symbol} */
 TreeOutline.Events = {
-    ElementAttached: "ElementAttached",
-    ElementExpanded: "ElementExpanded",
-    ElementCollapsed: "ElementCollapsed",
-    ElementSelected: "ElementSelected"
+    ElementAttached: Symbol("ElementAttached"),
+    ElementExpanded: Symbol("ElementExpanded"),
+    ElementCollapsed: Symbol("ElementCollapsed"),
+    ElementSelected: Symbol("ElementSelected")
 }
 
 TreeOutline.prototype = {
     _createRootElement: function()
     {
         this._rootElement = new TreeElement();
         this._rootElement.treeOutline = this;
         this._rootElement.root = true;
         this._rootElement.selectable = false;
         this._rootElement.expanded = true;
@@ skipping 1097 matching lines @@
     isEventWithinDisclosureTriangle: function(event)
     {
         // FIXME: We should not use getComputedStyle(). For that we need to get rid of using ::before for disclosure triangle. (http://webk.it/74446)
         var paddingLeftValue = window.getComputedStyle(this._listItemNode).paddingLeft;
         console.assert(paddingLeftValue.endsWith("px"));
         var computedLeftPadding = parseFloat(paddingLeftValue);
         var left = this._listItemNode.totalOffsetLeft() + computedLeftPadding;
         return event.pageX >= left && event.pageX <= left + TreeElement._ArrowToggleWidth && this._expandable;
     }
 }