Cancel client auth requests when not promptable.

chrome/test/data/extensions/api_test/xhr_fail/background.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 xhr = new XMLHttpRequest();
+  xhr.open("GET", config.customArg);
+  // The XHR is expected to fail.
+  xhr.onerror = function(ev) {
+    chrome.test.notifyPass();
+  };
+  xhr.onload = function(ev) {
+    chrome.test.notifyFail("XHR unexpectedly passed");
+  };
+  xhr.send();
+});