[MD History] Add shift-selection to the history lists.

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.
 
 Polymer({
   is: 'history-app',
 
   properties: {
     showSidebarFooter: Boolean,
 
@@ skipping 89 matching lines @@
       drawer.toggle();
   },
 
   /**
    * Listens for history-item being selected or deselected (through checkbox)
    * and changes the view of the top toolbar.
    * @param {{detail: {countAddition: number}}} e
    */
   checkboxSelected: function(e) {
     var toolbar = /** @type {HistoryToolbarElement} */ (this.$.toolbar);
-    toolbar.count += e.detail.countAddition;
+    toolbar.count = /** @type {HistoryListContainerElement} */ (this.$.history)
+                        .getSelectedItemCount();
   },
 
   /**
    * Listens for call to cancel selection and loops through all items to set
    * checkbox to be unselected.
    * @private
    */
   unselectAll: function() {
     var listContainer =
-        /** @type {HistoryListContainerElement} */ (this.$['history']);
+        /** @type {HistoryListContainerElement} */ (this.$.history);
     var toolbar = /** @type {HistoryToolbarElement} */ (this.$.toolbar);
     listContainer.unselectAllItems(toolbar.count);
     toolbar.count = 0;
   },
 
   deleteSelected: function() {
     this.$.history.deleteSelectedWithPrompt();
   },
 
   /**
@@ skipping 144 matching lines @@
     return selectedPage;
   },
 
   /** @private */
   closeDrawer_: function() {
     var drawer = this.$$('#drawer');
     if (drawer)
       drawer.close();
   },
 });