[MD Bookmarks] Persist collapsed folders between page loads.

chrome/test/data/webui/md_bookmarks/test_store.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.
 
 cr.define('bookmarks', function() {
   var TestStore = function(data) {
     this.data = Object.assign(bookmarks.util.createEmptyState(), data);
     this.lastAction_ = null;
     this.observers_ = [];
+    this.acceptInit_ = false;
   };
 
   TestStore.prototype = {
     addObserver: function(client) {
       this.observers_.push(client);
     },
 
-    init: function() {},
+    init: function(state) {
+      if (this.acceptInit_)
+        this.data = state;
+    },
 
     removeObserver: function(client) {},
 
     isInitialized: function() {
       return true;
     },
 
     handleAction: function(action) {
       this.lastAction_ = action;
     },
 
     get lastAction() {
       return this.lastAction_;
     },
 
     notifyObservers: function() {
       // TODO(tsergeant): Revisit how state modifications work in UI tests.
       // We don't want tests to worry about modifying the whole state tree.
       // Instead, we could perform a deep clone in here to ensure that every
       // StoreClient is updated.
       this.observers_.forEach((client) => client.onStateChanged(this.data));
     },
+
+    // Call in order to accept data from an init call to the TestStore.
+    acceptInit: function() {
+      this.acceptInit_ = true;
+    },
   };
 
   return {
     TestStore: TestStore,
   };
 });