[MD History] Make forward/backward work in grouped mode.

chrome/browser/resources/md_history/app.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.
 
 cr.define('md_history', function() {
   var lazyLoadPromise = null;
   function ensureLazyLoaded() {
     if (!lazyLoadPromise) {
       lazyLoadPromise = new Promise(function(resolve, reject) {
         Polymer.Base.importHref(
@@ skipping 32 matching lines @@
       type: Object,
       value: function() {
         return {
           // Whether the most recent query was incremental.
           incremental: false,
           // A query is initiated by page load.
           querying: true,
           queryingDisabled: false,
           _range: HistoryRange.ALL_TIME,
           searchTerm: '',
-          // TODO(calamity): Make history toolbar buttons change the offset
           groupedOffset: 0,
 
           set range(val) { this._range = Number(val); },
           get range() { return this._range; },
         };
       }
     },
 
     /** @type {!QueryResult} */
     queryResult_: {
@@ skipping 331 matching lines @@
             break;
         }
         break;
     }
 
     md_history.BrowserService.getInstance().recordHistogram(
       'History.HistoryPageView', histogramValue, HistoryPageViewHistogram.END
     );
   },
 });