[MD Bookmarks] Add keyboard navigation to sidebar.

chrome/test/data/webui/md_bookmarks/test_util.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.
 
 /**
  * Replace the current body of the test with a new element.
  * @param {Element} element
  */
 function replaceBody(element) {
   PolymerTest.clearBody();
@@ skipping 96 matching lines @@
 
   if (config) {
     for (var key in config)
       props[key] = config[key];
   }
 
   element.dispatchEvent(new MouseEvent('mousedown', props));
   element.dispatchEvent(new MouseEvent('mouseup', props));
   element.dispatchEvent(new MouseEvent('click', props));
 }
+
+/**
+ * Returns a folder node beneath |rootNode| which matches |id|.
+ * @param {BookmarksFolderNodeElement} rootNode
+ * @param {string} id
+ * @return {BookmarksFolderNodeElement}
+ */
+function findFolderNode(rootNode, id) {
+  var nodes = [rootNode];
+  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)});
+  }
+}