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

chrome/browser/resources/md_history/list_container.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-list-container',
 
   properties: {
     // The path of the currently selected page.
     selectedPage_: String,
@@ skipping 68 matching lines @@
     }
 
     var maxResults =
       queryState.range == HistoryRange.ALL_TIME ? RESULTS_PER_PAGE : 0;
     chrome.send('queryHistory', [
       queryState.searchTerm, queryState.groupedOffset, queryState.range,
       lastVisitTime, maxResults
     ]);
   },
 
+  /** @return {number} */
+  getSelectedItemCount: function() {
+    return this.getSelectedList_().selectedPaths.size;
+  },
+
   unselectAllItems: function(count) {
     this.getSelectedList_().unselectAllItems(count);
   },
 
   /**
    * Delete all the currently selected history items. Will prompt the user with
    * a dialog to confirm that the deletion should be performed.
    */
   deleteSelectedWithPrompt: function() {
     if (!loadTimeData.getBoolean('allowDeletingHistory'))
@@ skipping 94 matching lines @@
   },
 
   /**
    * @return {HTMLElement}
    * @private
    */
   getSelectedList_: function() {
     return this.$.content.selectedItem;
   },
 });