[MD Bookmarks] Add keyboard navigation to sidebar.

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.
 
 suiteSetup(function() {
   cr.define('bookmarks', function() {
     var TestStore = function(data) {
       bookmarks.Store.call(this);
       this.data_ = Object.assign(bookmarks.util.createEmptyState(), data);
       this.initialized_ = true;
 
       this.lastAction_ = null;
       this.acceptInit_ = false;
       this.enableReducers_ = false;
     };
 
     TestStore.prototype = {
       __proto__: bookmarks.Store.prototype,
 
       init: function(state) {
         if (this.acceptInit_)
           bookmarks.Store.prototype.init.call(this, state);
       },
 
       get lastAction() {
         return this.lastAction_;
       },
 
+      resetLastAction() {
+        this.lastAction_ = null;
+      },
+
       get data() {
         return this.data_;
       },
 
       set data(newData) {
         this.data_ = newData;
       },
 
       /**
        * Enable or disable calling bookmarks.reduceAction for each action.
@@ skipping 28 matching lines @@
         this.acceptInit_ = true;
         this.initialized_ = false;
       },
     };
 
     return {
       TestStore: TestStore,
     };
   });
 });