DevTools: Fix closure compiler error.

Source/core/inspector/InjectedScriptExterns.js

 /*
  * Copyright (C) 2012 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 161 matching lines @@
  */
 CallSite.prototype.getFileName = function() { }
 /**
  * @return {number}
  */
 CallSite.prototype.getLineNumber = function() { }
 /**
  * @return {number}
  */
 CallSite.prototype.getColumnNumber = function() { }
+
+// FIXME: Remove once ES6 is supported natively by JS compiler.
+
+/** @typedef {?} */
+var symbol;
+
+/**
+ * @param {string} description
+ * @return {symbol}
+ */
+function Symbol(description) {}