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

chrome/browser/resources/md_bookmarks/list.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_action_menu/cr_action_menu.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_lazy_render/cr_lazy_render.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button-light.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/shadow.html">
 <link rel="import" href="chrome://bookmarks/edit_dialog.html">
 <link rel="import" href="chrome://bookmarks/item.html">
 <link rel="import" href="chrome://bookmarks/shared_style.html">
 <link rel="import" href="chrome://bookmarks/store_client.html">
 <link rel="import" href="chrome://bookmarks/util.html">
@@ skipping 17 matching lines @@
       .centered-message {
         align-items: center;
         color: #6e6e6e;
         display: flex;
         font-size: 14px;
         font-weight: 500;
         height: 100%;
         justify-content: center;
       }
     </style>
-    <dialog is="cr-action-menu" id="dropdown">
+    <dialog is="cr-action-menu" id="dropdown" on-mousedown="onMenuMousedown_">
       <button class="dropdown-item" on-tap="onEditTap_">
         [[getEditActionLabel_(menuItem_)]]
       </button>
       <button class="dropdown-item" on-tap="onCopyURLTap_"
           hidden$="[[!menuItem_.url]]">
         $i18n{menuCopyURL}
       </button>
       <button class="dropdown-item" on-tap="onDeleteTap_">
          $i18n{menuDelete}
       </button>
     </dialog>
     <div id="bookmarksCard" hidden$="[[isEmptyList_(displayedList_.length)]]">
       <template is="dom-repeat" items="[[displayedList_]]" as="id">
         <bookmarks-item item-id="[[id]]" draggable="true">
         </bookmarks-item>
       </template>
     </div>
     <div id="message" class="centered-message"
         hidden$="[[!isEmptyList_(displayedList_.length)]]">
       [[emptyListMessage_(searchTerm_)]]
     </div>
     <template is="cr-lazy-render" id="editDialog">
       <bookmarks-edit-dialog></bookmarks-edit-dialog>
     </template>
   </template>
   <script src="chrome://bookmarks/list.js"></script>
 </dom-module>