[MD Bookmarks] Add keyboard navigation to sidebar.

chrome/test/data/webui/md_bookmarks/dnd_manager_test.js

 // Copyright 2017 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.
 
 suite('drag and drop', function() {
   var app;
   var list;
-  var sidebar;
+  var rootFolderNode;
   var store;
   var dndManager;
   var draggedIds;
 
   var DRAG_STYLE = {
     NONE: 0,
     ON: 1,
     ABOVE: 2,
     BELOW: 3,
   };
 
   function getFolderNode(id) {
-    var nodes = [sidebar];
-    var node;
-    while (nodes.length) {
-      node = nodes.pop();
-      if (node.itemId == id)
-        return node;
-
-      node.root.querySelectorAll('bookmarks-folder-node')
-          .forEach((x) => {nodes.unshift(x)});
-    }
+    return findFolderNode(rootFolderNode, id);
   }
 
   function getListItem(id) {
     var items = list.root.querySelectorAll('bookmarks-item');
     for (var i = 0; i < items.length; i++) {
       if (items[i].itemId == id)
         return items[i];
     }
   }
 
@@ skipping 59 matching lines @@
     });
     bookmarks.Store.instance_ = store;
 
     chrome.bookmarkManagerPrivate.startDrag = function(nodes, isTouch) {
       draggedIds = nodes;
     };
 
     app = document.createElement('bookmarks-app');
     replaceBody(app);
     list = app.$$('bookmarks-list');
-    sidebar = app.$$('bookmarks-folder-node');
+    rootFolderNode = app.$$('bookmarks-folder-node');
     dndManager = app.dndManager_;
     dndManager.dropIndicator_.disableTimeoutForTesting();
     Polymer.dom.flush();
   });
 
   test('dragInfo isDraggingFolderToDescendant', function() {
     var dragInfo = new bookmarks.DragInfo();
     var nodes = store.data.nodes;
     dragInfo.handleChromeDragEnter(createDragData(['11']));
     assertTrue(dragInfo.isDraggingFolderToDescendant('111', nodes));
@@ skipping 353 matching lines @@
 
     dispatchDragEvent('dragstart', dragElement);
     dndManager.dragInfo_.handleChromeDragEnter(createDragData(draggedIds));
 
     dispatchDragEvent('dragover', dragTarget);
     assertEquals(
         DropPosition.NONE, dndManager.calculateValidDropPositions_(dragTarget));
     assertDragStyle(dragTarget, DRAG_STYLE.NONE);
   });
 });