[MD Bookmarks] Fix offscreen menu button activation.

chrome/browser/resources/md_bookmarks/list.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-list/iron-list.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/command_manager.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 19 matching lines @@
         align-items: center;
         color: #6e6e6e;
         display: flex;
         font-size: 14px;
         font-weight: 500;
         height: 100%;
         justify-content: center;
       }
     </style>
     <iron-list items="[[displayedList_]]" id="bookmarksCard"
         hidden$="[[isEmptyList_(displayedList_.length)]]">

[tsergeant, 2017/06/21 02:46:18]

I think we can do this without adding new listeners/bindings inside the
iron-list template (meaning slightly better performance, less coupling to
iron-list).

1) Add listener for 'open-item-menu' to the <bookmarks-list>
2) Inside the listener, get the <bookmarks-item> element from e.path
3) Get itemElement.itemId
4) Use displayedIds_.indexOf to determine the index
5) Scroll the iron-list to that index if necessary.

WDYT?

       <template>
         <bookmarks-item item-id="[[item.id]]" draggable="true"
-            tabindex$="[[tabIndex]]" iron-list-tab-index="[[tabIndex]]">
+            tabindex$="[[tabIndex]]" iron-list-tab-index="[[tabIndex]]"
+            index="[[index]]" on-open-item-menu="onOpenItemMenu_">
         </bookmarks-item>
       </template>
     </iron-list>
     <div id="message" class="centered-message"
         hidden$="[[!isEmptyList_(displayedList_.length)]]">
       [[emptyListMessage_(searchTerm_)]]
     </div>
   </template>
   <script src="chrome://bookmarks/list.js"></script>
 </dom-module>