[MD Bookmarks] Fix offscreen menu button activation.

chrome/browser/resources/md_bookmarks/list.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.
 
 Polymer({
   is: 'bookmarks-list',
 
   behaviors: [
     bookmarks.StoreClient,
   ],
@@ skipping 114 matching lines @@
 
   /**
    * @param{HTMLElement} el
    * @private
    */
   getIndexForItemElement_: function(el) {
     return this.$.bookmarksCard.modelForElement(el).index;
   },
 
   /**
+   * @param {Event} e
+   * @private
+   */
+  onOpenItemMenu_: function(e) {
+    var index = this.displayedIds_.indexOf(
+        /** @type {BookmarksItemElement} */ (e.target).itemId);
+    var list = this.$.bookmarksCard;
+    // If the item is not visible, scroll to it before rendering the menu.
+    if (index < list.firstVisibleIndex || index > list.lastVisibleIndex)
+      list.scrollToIndex(index);
+  },
+
+  /**
    * @param {KeyboardEvent} e
    * @private
    */
   onItemKeydown_: function(e) {
     var handled = true;
     var list = this.$.bookmarksCard;
     var focusMoved = false;
     var focusedIndex =
         this.getIndexForItemElement_(/** @type {HTMLElement} */ (e.target));
     var oldFocusedIndex = focusedIndex;
@@ skipping 54 matching lines @@
 
     if (!handled) {
       handled = bookmarks.CommandManager.getInstance().handleKeyEvent(
           e, this.getState().selection.items);
     }
 
     if (handled)
       e.stopPropagation();
   },
 });