Cancel client auth requests when not promptable.

chrome/test/data/extensions/api_test/background_xhr/test_http_auth.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.
+
+chrome.test.getConfig(function(config) {
+  var url = 'http://localhost:' + config.spawnedTestServer.port + '/auth-basic';
+
+  chrome.webRequest.onCompleted.addListener(function(details) {
+    chrome.test.assertEq(-1, details.tabId);
+    chrome.test.assertEq(url, details.url);
+    chrome.test.assertEq("xmlhttprequest", details.type);
+    chrome.test.assertEq(401, details.statusCode);
+
+    chrome.test.notifyPass();
+  }, { urls: [url], types: ["xmlhttprequest"] });
+
+  chrome.runtime.sendMessage({type: 'xhr', method: 'GET', url: url});
+});