[MD Bookmarks] Persist collapsed folders between page loads.

chrome/browser/resources/md_bookmarks/constants.js

Index: chrome/browser/resources/md_bookmarks/constants.js
diff --git a/chrome/browser/resources/md_bookmarks/constants.js b/chrome/browser/resources/md_bookmarks/constants.js
new file mode 100644
index 0000000000000000000000000000000000000000..f6757aae2ff158c0c37d2b2940a6d561492d1e1e
--- /dev/null
+++ b/chrome/browser/resources/md_bookmarks/constants.js
@@ -0,0 +1,22 @@
+// 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.
+
+/**
+ * Enumeration of valid drop locations relative to an element. These are
+ * bit masks to allow combining multiple locations in a single value.
+ * @enum {number}
+ * @const
+ */
+var DropPosition = {
+  NONE: 0,
+  ABOVE: 1,
+  ON: 2,
+  BELOW: 4,
+};
+
+/** @const */
+var LOCAL_STORAGE_CLOSED_FOLDERS_KEY = 'bookmarkManagerClosedState';
+
+/** @const */
+var ROOT_NODE_ID = '0';