[MD Bookmarks] Add keyboard navigation to sidebar.

chrome/browser/resources/md_bookmarks/folder_node.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button-light.html">
 <link rel="import" href="chrome://bookmarks/actions.html">
 <link rel="import" href="chrome://bookmarks/icons.html">
 <link rel="import" href="chrome://bookmarks/shared_style.html">
 <link rel="import" href="chrome://bookmarks/store_client.html">
 
 <dom-module id="bookmarks-folder-node">
@@ skipping 58 matching lines @@
         transform: initial;
       }
 
       [no-children] {
         -webkit-padding-start: 52px; /* The width of the arrow and its margin */
       }
     </style>
 
     <div id="container" class="v-centered" on-tap="selectFolder_"
         draggable="[[!isTopLevelFolder_(depth)]]"
+        tabindex$="[[getTabIndex_(isSelectedFolder_)]]"
         hidden="[[isRootFolder_(depth)]]">
       <template is="dom-if" if="[[hasChildFolder_(item_.children)]]">
         <button is="paper-icon-button-light" id="arrow"
-            is-open$="[[!isClosed_]]" on-tap="toggleFolder_">
+            is-open$="[[!isClosed_]]" on-tap="toggleFolder_" tabindex="">

[tsergeant, 2017/05/03 03:56:38]

With the change to paper-icon-button-light, tabindex doesn't seem to take effect
like it used to. You can now tab through to each of the buttons.

Blergh. Any ideas how we can fix that? Maybe we'll need to preventDefault on
mousedown or similar.

[calamity, 2017/05/03 05:13:56]

Made mousedown preventDefault.

           <iron-icon icon="cr:arrow-drop-down"></iron-icon>
         </button>
       </template>
       <div id="folder-label" class="v-centered">
         <iron-icon icon="[[getFolderIcon_(isSelectedFolder_)]]"
             no-children$="[[!hasChildFolder_(item_.children)]]">
         </iron-icon>
         <div class="menu-label">[[item_.title]]</div>
       </div>
     </div>
     <div id="descendants" hidden$="[[isClosed_]]">
       <template is="dom-repeat"
           items="[[item_.children]]"
           as="child"
           filter="isFolder_">
         <bookmarks-folder-node item-id="[[child]]"
             depth="[[getChildDepth_(depth)]]">
         </bookmarks-folder-node>
       </template>
     </div>
   </template>
   <script src="chrome://bookmarks/folder_node.js"></script>
 </dom-module>