[MD History] Factor out a common HistoryListBehavior.

chrome/browser/resources/md_history/history_toolbar.js

 // Copyright 2015 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.
 
 Polymer({
   is: 'history-toolbar',
   properties: {
     // Number of history items currently selected.
+    // TODO(calamity): bind this to
+    // listContainer.selectedItem.selectedPaths.length.
     count: {
       type: Number,
       value: 0,
       observer: 'changeToolbarView_'
     },
 
     // True if 1 or more history items are selected. When this value changes
     // the background colour changes.
     itemsSelected_: {
       type: Boolean,
@@ skipping 109 matching lines @@
     // TODO(calamity): Fix the format of these dates.
     return loadTimeData.getStringF(
       'historyInterval', queryStartTime, queryEndTime);
   },
 
   /** @private */
   hasDrawerChanged_: function() {
     this.updateStyles();
   },
 });