chrome.settingsPrivate: Implement onPrefsChanged event handling.

chrome/test/data/extensions/api_test/settings_private/test.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.
 
 // This just tests the interface. It does not test for specific results, only
 // that callbacks are correctly invoked, expected parameters are correct,
 // and failures are detected.
 
-var kTestPrefName = 'test.foo_bar';
+var kTestPrefName = 'download.default_directory';
+var kTestPrefValue = '/Downloads';
 var kTestPageId = 'pageId';
 
 function callbackResult(result) {
   if (chrome.runtime.lastError)
     chrome.test.fail(chrome.runtime.lastError.message);
   else if (result == false)
     chrome.test.fail('Failed: ' + result);
 }
 
 var availableTests = [
   function setPref() {
     chrome.settingsPrivate.setPref(
         kTestPrefName,
-        true,
+        kTestPrefValue,
         kTestPageId,
         function(success) {
           callbackResult(success);
           chrome.test.succeed();
         });
   },
   function getPref() {
     chrome.settingsPrivate.getPref(
         kTestPrefName,
         function(value) {
           chrome.test.assertTrue(value !== null);
           callbackResult(true);
           chrome.test.succeed();
         });
   },
   function getAllPrefs() {
     chrome.settingsPrivate.getAllPrefs(
         function(prefs) {
           chrome.test.assertTrue(prefs.length > 0);
           callbackResult(true);
           chrome.test.succeed();
         });
   },
+  function onPrefsChanged() {
+    chrome.settingsPrivate.onPrefsChanged.addListener(function(prefs) {
+      chrome.test.assertTrue(prefs.length > 0);
+      chrome.test.assertEq(kTestPrefName, prefs[0].key);
+      chrome.test.assertEq(kTestPrefValue, prefs[0].value);
+      callbackResult(true);
+      chrome.test.succeed();
+    });
+
+    chrome.settingsPrivate.setPref(
+        kTestPrefName,
+        kTestPrefValue,
+        kTestPageId,
+        function() {});
+  },
 ];
 
 var testToRun = window.location.search.substring(1);
 chrome.test.runTests(availableTests.filter(function(op) {
   return op.name == testToRun;
 }));