[chrome.dial] Adds chrome.dial.fetchDeviceDecription API.

chrome/test/data/extensions/api_test/dial/experimental/fetch_device_description.js

+// Copyright (c) 2017 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.
+
+onload = function() {
+  chrome.test.runTests([
+    function fetchDeviceDecription() {
+      const fetchDeviceDescriptionCallback = result => {
+        chrome.test.assertNoLastError();
+        chrome.test.assertEq('object', typeof(result));
+        chrome.test.assertEq('http://127.0.0.1/apps', result.appUrl);
+        chrome.test.assertEq('<xml>testDescription</xml>',
+                             result.deviceDescription);
+        chrome.test.succeed();
+      };
+      chrome.dial.fetchDeviceDescription('testDevice',
+                                         fetchDeviceDescriptionCallback);
+    },
+    function fetchDeviceDecriptionFails() {
+      const fetchDeviceDescriptionCallbackFails = result => {
+        chrome.test.assertEq(undefined, result);
+        chrome.test.assertLastError('Device not found');
+        chrome.test.succeed();
+      };
+      chrome.dial.fetchDeviceDescription('unknownDevice',
+                                         fetchDeviceDescriptionCallbackFails);
+    }
+    // TODO(mfoltz): Test other fetch errors
+  ]);
+};