[MD Bookmarks] Persist collapsed folders between page loads.

chrome/test/data/webui/md_bookmarks/app_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('<bookmarks-app>', function() {
+  var app;
+  var store;
+
+  function resetStore() {
+    store = new bookmarks.TestStore({});
+    store.acceptInitOnce();
+    bookmarks.Store.instance_ = store;
+
+    chrome.bookmarks.getTree = function(fn) {
+      fn([
+        createFolder(
+            '0',
+            [
+              createFolder(
+                  '1',
+                  [
+                    createFolder('11', []),
+                  ]),
+            ]),
+      ]);
+    };
+  }
+
+  setup(function() {
+    resetStore();
+
+    app = document.createElement('bookmarks-app');
+    replaceBody(app);
+  });
+
+  teardown(function() {
+    window.localStorage.clear();
+  });
+
+  test('write and load closed folder state', function() {
+    var closedFolders = {'1': true};
+    store.data.closedFolders = closedFolders;
+    store.notifyObservers();
+
+    // Ensure closed folders are written to local storage.
+    assertDeepEquals(
+        JSON.stringify(closedFolders),
+        window.localStorage[LOCAL_STORAGE_CLOSED_FOLDERS_KEY]);
+
+    resetStore();
+    app = document.createElement('bookmarks-app');
+    replaceBody(app);
+
+    // Ensure closed folders are read from local storage.
+    assertDeepEquals(closedFolders, store.data.closedFolders);
+  });
+});