DevTools: Fix closure compiler error.

Source/core/inspector/InjectedScriptExterns.js

Index: Source/core/inspector/InjectedScriptExterns.js
diff --git a/Source/core/inspector/InjectedScriptExterns.js b/Source/core/inspector/InjectedScriptExterns.js
index 55dce746f80078198bb87b22fb25f4549c0ba2ba..4c5ecc9c640c45dc74f1f51e4bebfbd88b47baf4 100644
--- a/Source/core/inspector/InjectedScriptExterns.js
+++ b/Source/core/inspector/InjectedScriptExterns.js
@@ -179,3 +179,14 @@ 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) {}