Don't add extra items to sheriff-o-matic history.

Tools/GardeningServer/ui/ct-embedded-flakiness-dashboard.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-embedded-flakiness-dashboard" attributes="test tree">
   <template>
     <style>
       :host {
@@ skipping 14 matching lines @@
       ready: function() {
         window.addEventListener('message', function(event) {
           if (event.data.command != 'heightChanged')
             return;
 
           this.$.iframe.style.height = event.data.height + 'px';
         }.bind(this));
       },
 
       testChanged: function() {
-        this.$.iframe.src = this.test.embeddedFlakinessDashboardURL(this.tree);
+        // Use location.replace so we don't create extraneous history events.
+        this.$.iframe.contentWindow.location.replace(this.test.embeddedFlakinessDashboardURL(this.tree));
       },
 
     });
   </script>
 </polymer-element>