MD Bookmarks: Fix Meta-down not triggering the 'Open' command on Mac

chrome/test/data/webui/md_bookmarks/command_manager_test.js

 // Copyright 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.
 
 suite('<bookmarks-command-manager>', function() {
   var commandManager;
   var store;
   var lastCommand;
   var lastCommandIds;
 
@@ skipping 364 matching lines @@
 
     assertOpenedTabs(['http://111/', 'http://12/']);
   });
 
   test('control-double click opens full selection', function() {
     customClick(items[0]);
     simulateDoubleClick(items[2], {ctrlKey: true});
 
     assertOpenedTabs(['http://111/', 'http://13/']);
   });
+
+  test('meta-down triggers Open on Mac', function() {
+    if (!cr.isMac)
+      return;
+
+    customClick(items[0]);
+    MockInteractions.pressAndReleaseKeyOn(items[0], 0, 'meta', 'ArrowDown');
+    assertEquals('11', store.data.selectedFolder);
+  });
 });