[MD Bookmarks] Right click on bookmark items open context menu.

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

 // Copyright 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.
 
 suite('<bookmarks-item>', function() {
   var item;
   var store;
   var TEST_ITEM = createItem('0');
 
   setup(function() {
@@ skipping 30 matching lines @@
     assertFalse(item.$['folder-icon'].hidden);
     assertTrue(item.$.icon.hidden);
   });
 
   test('pressing the menu button selects the item', function() {
     MockInteractions.tap(item.$$('.more-vert-button'));
     assertDeepEquals(
         bookmarks.actions.selectItem('2', false, false, store.data),
         store.lastAction);
   });
+
+  test('context menu selects item if unselected', function() {
+    item.isSelectedItem_ = true;
+    item.dispatchEvent(new MouseEvent('contextmenu'));
+    assertEquals(null, store.lastAction);
+
+    item.isSelectedItem_ = false;
+    item.dispatchEvent(new MouseEvent('contextmenu'));
+    assertDeepEquals(
+        bookmarks.actions.selectItem('2', false, false, store.data),
+        store.lastAction);
+  });
 });