[MD History] Make history-list:historyData private.

chrome/test/data/webui/md_history/history_toolbar_test.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.history_toolbar_test', function() {
   function registerTests() {
     suite('history-toolbar', function() {
       var element;
       var toolbar;
       var TEST_HISTORY_RESULTS;
@@ skipping 48 matching lines @@
                 'example.com');
             done();
           });
         });
 
         element.$.sharedMenu.itemData = {domain: 'example.com'};
         MockInteractions.tap(element.$.menuMoreButton);
       });
 
       teardown(function() {
-        element.historyData = [];
+        element.historyData_ = [];
         element.searchedTerm = '';
         registerMessageCallback('queryHistory', this, undefined);
         toolbar.count = 0;
       });
     });
   }
   return {
     registerTests: registerTests
   };
 });