[MD Bookmarks] Add keyboard navigation to sidebar.

chrome/browser/resources/md_bookmarks/folder_node.html

Index: chrome/browser/resources/md_bookmarks/folder_node.html
diff --git a/chrome/browser/resources/md_bookmarks/folder_node.html b/chrome/browser/resources/md_bookmarks/folder_node.html
index c9bb16c4ecec230448e520615518a6b11159334f..72b7ccd9f5da417212d13e87a028722f0334fe1c 100644
--- a/chrome/browser/resources/md_bookmarks/folder_node.html
+++ b/chrome/browser/resources/md_bookmarks/folder_node.html
@@ -76,10 +76,11 @@
 
     <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>