Add failed_once category to Sheriff-O-Matic.

Tools/GardeningServer/model/test/ct-failure-group-tests.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.
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 <link rel="import" href="../ct-failure-group.html">
 
 <script>
 (function () {
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   describe('category', function() {
     it('should be "default" by default', function() {
       var group = new CTFailureGroup([], undefined);
       assert.equal(group.category, 'default');
     });
 
     it('should be "snoozed" when snoozed', function() {
       var group = new CTFailureGroup([newFailureWithAnnotation({snoozeTime: Date.now() + 1000 * 1000})], undefined);
       assert.equal(group.category, 'snoozed');
     });
+
+    it('should be "failed_once" when there is only one failure', function() {
+      var failure = newFailureWithAnnotation();
+      failure.resultNodesByBuilder = {some_key: {failingBuildCount: 1}}
+      var group = new CTFailureGroup([failure], undefined);
+      assert.equal(group.category, 'failed_once');
+    });
+
+    it('should not be "failed_once" when there is more than one failure', function() {
+      var failure = newFailureWithAnnotation();
+      failure.resultNodesByBuilder = {some_key: {failingBuildCount: 1},
+                                      other_key: {failingBuildCount: 1}}
+      var group = new CTFailureGroup([failure], undefined);
+      assert.equal(group.category, 'default');
+    });
   });
 
   describe('snooze', function() {
     it('should set isSnoozed', function(done) {
       var group = new CTFailureGroup([newFailureWithAnnotation()], undefined);
       group.snoozeUntil(Date.now() + 1000 * 1000).then(function() {
         assert.isTrue(group.isSnoozed);
         done();
       });
     });
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             done();
           });
         });
       });
     });
   });
 });
 
 })()
 </script>