[MD Bookmarks] Add keyboard navigation to sidebar.

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

Index: chrome/test/data/webui/md_bookmarks/test_util.js
diff --git a/chrome/test/data/webui/md_bookmarks/test_util.js b/chrome/test/data/webui/md_bookmarks/test_util.js
index 090758749019d447ceb5c8d237dabe997b2fdeac..f1318eb760fca901094fcbc5c03c53c405a4284a 100644
--- a/chrome/test/data/webui/md_bookmarks/test_util.js
+++ b/chrome/test/data/webui/md_bookmarks/test_util.js
@@ -114,3 +114,22 @@ function customClick(element, config) {
   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)});
+  }
+}