Ensure that sendMessage's callback is called.

chrome/test/data/extensions/api_test/messaging/connect_crash/page.js

 // Copyright 2015 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.
 
 var port = chrome.runtime.connect();
 port.onDisconnect.addListener(function() {
   chrome.test.fail('onDisconnect should not be triggered because the ' +
      'background page exists and the tab should have been crashed');
 });
 
 var ref;
 chrome.runtime.onMessage.addListener(function(msg, sender, sendResponse) {
   chrome.test.assertEq('Rob says hi', msg);
   port.postMessage('is_ready_to_crash');
   // Keep the callback around to avoid test flakiness due to GC.
   ref = sendResponse;
 
-  // TODO(robwu): Remove the following line once crbug.com/439780 is fixed.
-  // (the response callback is not automatically invoked when the tab crashes).
-  sendResponse();
-
   // Keep the port open - do not send a response.
   return true;
 });