Take two at fixing duplicate alert failures.

Tools/GardeningServer/model/ct-step-failure.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.
 -->
 
 <script>
 function CTStepFailure(step, reason, resultsByBuilder) {
-  this.key = step + '::' + reason;
+  this.key = step + '::'
+
+  // The key needs to be unique, so disambiguate null reason
+  // failures by their builders, but don't do regular failures
+  // by builders because we want failures with reasons to update
+  // the existing failure group instead of causing the group to
+  // get recreated since throwing it away loses in app state.
+  if (reason)
+    this.key += reason;
+  else
+    this.key += Object.keys(resultsByBuilder).join('::');
+
   this.step = step;
   // FIXME: Rename this to reason.
   this.testName = reason;
   // Maps a builder_name (e.g. "Mac ASan Tests (1)")
   // to the details of the failure on that builder.
   this.resultNodesByBuilder = resultsByBuilder;
 }
 
 CTStepFailure.prototype.isTreeCloser = function() {
   for (var builder in this.resultNodesByBuilder) {
@@ skipping 45 matching lines @@
 
 // FIXME: This should just be provided as part of the alerts feed.
 CTStepFailure.createKey = function(alert) {
   function normalize(str) {
     str = str == null ? '' : String(str);
     return str.replace(/:/g, '_');
   }
   return [alert.master_url, alert.builder_name, alert.failing_build, alert.step_name, alert.reason].map(normalize).join('::');
 }
 </script>