Cancel client auth requests when not promptable.

chrome/test/data/extensions/api_test/background_xhr/background.js

Index: chrome/test/data/extensions/api_test/background_xhr/background.js
diff --git a/chrome/test/data/extensions/api_test/background_xhr/background.js b/chrome/test/data/extensions/api_test/background_xhr/background.js
new file mode 100644
index 0000000000000000000000000000000000000000..bd147e50af30d748b22f78928c4af48085feb972
--- /dev/null
+++ b/chrome/test/data/extensions/api_test/background_xhr/background.js
@@ -0,0 +1,13 @@
+// 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.runtime.onMessage.addListener(function(message, sender, sendResponse) {
+  if (message.type == 'xhr') {
+    var xhr = new XMLHttpRequest();
+    xhr.open(message.method, message.url);
+    xhr.send();
+  } else {
+    console.error('Unknown message: ' + JSON.stringify(message));
+  }
+});