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

chrome/test/data/webui/md_history/history_toolbar_test.js

Index: chrome/test/data/webui/md_history/history_toolbar_test.js
diff --git a/chrome/test/data/webui/md_history/history_toolbar_test.js b/chrome/test/data/webui/md_history/history_toolbar_test.js
index b2c9cb025246925571b733561fe7b67e25c279fe..56148125a3b96c7ac01f6c438ddae5ef819f5579 100644
--- a/chrome/test/data/webui/md_history/history_toolbar_test.js
+++ b/chrome/test/data/webui/md_history/history_toolbar_test.js
@@ -66,7 +66,7 @@ cr.define('md_history.history_toolbar_test', function() {
       });
 
       teardown(function() {
-        element.historyData = [];
+        element.historyData_ = [];
         element.searchedTerm = '';
         registerMessageCallback('queryHistory', this, undefined);
         toolbar.count = 0;