Add failed_once category to Sheriff-O-Matic.

Tools/GardeningServer/ui/ct-unexpected-failures.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.
 -->
 
 <link rel="import" href="../bower_components/paper-button/paper-button.html">
 <link rel="import" href="../model/tree-status.html">
 <link rel="import" href="ct-failure-stream.html">
 <link rel="import" href="ct-last-updated.html">
@@ skipping 25 matching lines @@
       }
     </style>
     <div class="toolbar">
       <ct-revision-details id="revisionDetails" builderLatestRevisions="{{ failures.builderLatestRevisions }}" commitLog="{{ commitLog }}" tree="{{ tree }}"></ct-revision-details>
       <a href="https://code.google.com/p/chromium/wiki/UsefulURLs">Useful URLs</a>
     </div>
     <ct-tree-status status="{{ treeStatuses['chromium'] }}" state="{{ treeStatuses['chromium'].status }}"></ct-tree-status>
     <ct-tree-status status="{{ treeStatuses['blink'] }}" state="{{ treeStatuses['blink'].status }}"></ct-tree-status>
     <ct-party-time failures="{{ failures.failures[tree].length == 0 }}"></ct-party-time>
     <ct-failure-stream category="default" groups="{{ failures.failures[tree] }}" commitLog="{{ commitLog }}" tree="{{ tree }}"></ct-failure-stream>
+    Failures that have only happened once (on one bot):
+    <ct-failure-stream category="failed_once" groups="{{ failures.failures[tree] }}" commitLog="{{ commitLog }}" tree="{{ tree }}"></ct-failure-stream>
+    Snoozed failures:
     <ct-failure-stream category="snoozed" groups="{{ failures.failures[tree] }}" commitLog="{{ commitLog }}" tree="{{ tree }}"></ct-failure-stream>
   </template>
   <script>
   (function() {
     Polymer({
       tree: '',
 
       created: function() {
         this.treeStatuses = {};
         var projects = ['chromium', 'blink'];
         for (var i = 0; i < projects.length; i++) {
           this.treeStatuses[projects[i]] = new TreeStatus(projects[i]);
         }
       },
 
       attached: function() {
         this.update();
       },
 
       update: function() {
         // FIXME: These shouldn't update if there's already an update in progress.
         Object.keys(this.treeStatuses, function(tree, status) {
           status.update();
         });
       },
     });
   })();
   </script>
 </polymer-element>