[MD Bookmarks] Fix offscreen menu button activation.

chrome/browser/resources/md_bookmarks/list.html

Index: chrome/browser/resources/md_bookmarks/list.html
diff --git a/chrome/browser/resources/md_bookmarks/list.html b/chrome/browser/resources/md_bookmarks/list.html
index 00c8ceaf67452bd3ccb2fc9d4e40cf42d39f8198..ebd2f0205304a5c56e4e3b7e29e18178c4f6aab3 100644
--- a/chrome/browser/resources/md_bookmarks/list.html
+++ b/chrome/browser/resources/md_bookmarks/list.html
@@ -40,7 +40,8 @@
         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>