[MD Bookmarks] Add skeleton for MD Bookmarks.

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 primary element of the test with a new element. Useful
+ * as an alternative to PolymerTest.clearBody() which preserves styling.
+ * @param {Element} element
+ */
+function replaceBody(element) {
+  var body = document.body;
+  while (body.children.length > 0)
+    body.removeChild(body.children[0]);
+  body.appendChild(element);
+}