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

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

+// Copyright (c) 2016 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() {
+      var fetchDeviceDescriptionCallback = function(result) {
+        chrome.test.assertNoLastError();
+        chrome.test.assertTrue(typeof(result) == 'object');
+        chrome.test.assertTrue(result.appUrl == 'http://127.0.0.1/apps');
+        chrome.test.assertTrue(result.deviceDescription ==
+                               '<xml>testDescription</xml>');
+        chrome.test.succeed();
+      };
+      chrome.dial.fetchDeviceDescription('testDevice',
+                                         fetchDeviceDescriptionCallback);
+    }
+  ]);
+};