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

chrome/test/data/webui/md_history/history_item_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_item_test', function() {
   function registerTests() {
     suite('history-item', function() {
       var element;
       var TEST_HISTORY_RESULTS;
       var SEARCH_HISTORY_RESULTS;
@@ skipping 48 matching lines @@
           done();
         });
       });
 
       test('separator insertion after deletion', function(done) {
         element.addNewResults(TEST_HISTORY_RESULTS);
         flush(function() {
           var items =
               Polymer.dom(element.root).querySelectorAll('history-item');
 
-          element.removeDeletedHistory_([element.historyData[3]]);
-          assertEquals(5, element.historyData.length);
+          element.removeDeletedHistory_([element.historyData_[3]]);
+          assertEquals(5, element.historyData_.length);
 
           // Checks that a new time gap separator has been inserted.
           assertTrue(items[2].hasTimeGap);
 
-          element.removeDeletedHistory_([element.historyData[3]]);
+          element.removeDeletedHistory_([element.historyData_[3]]);
 
           // Checks time gap separator is removed.
           assertFalse(items[2].hasTimeGap);
           done();
         });
       });
 
       teardown(function() {
-        element.historyData = [];
+        element.historyData_ = [];
         element.searchedTerm = '';
       });
     });
   }
   return {
     registerTests: registerTests
   };
 });