Suppress the exception thrown when responding to message with no response callback.

chrome/test/data/extensions/hotword/test.js

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Send a WAKE_UP command to the hotword helper extension.
+chrome.runtime.sendMessage(
+    'dnhpdliibojhegemfjheidglijccjfmc',
+    {type: 'wu'});
+
+// Assume messages are delivered in-order. In this case, this message will be
+// handled after the previous one, and indicates the test case has completed.
+chrome.runtime.sendMessage(
+    'dnhpdliibojhegemfjheidglijccjfmc',
+    {type: 'wu'},
+    function() {
+      chrome.test.sendMessage("done");
+    });