Fix private style in ct-commit-data.html.

Tools/GardeningServer/ui/ct-commit-data.html

 <!--
 Copyright 2014 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-element name="ct-commit-data" attributes="first last data">
   <script>
     Polymer({
       data: [],
       first: 0,
       last: 0,
 
       firstChanged: function() {
-        this.update_();
+        this._update();
       },
 
       lastChanged: function() {
-        this.update_();
+        this._update();
       },
 
-      update_: function() {
+      _update: function() {
         this.data = model.commitDataListForRevisionRange(this.first, this.last);
       },
     });
   </script>
 </polymer-element>