Expose the chromium waterfall in sheriff-o-matic.

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">
+<polymer-element name="ct-embedded-flakiness-dashboard" attributes="test tree">
   <template>
     <style>
       :host {
         display: block;
       }
       #iframe {
         border: none;
         width: 100%;
       }
     </style>
     <iframe id="iframe"></iframe>
   </template>
   <script>
     Polymer({
       test: "",
+      tree: "",
 
       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.$.iframe.src = ui.urlForEmbeddedFlakinessDashboard(this.test.testName, this.test.step);
+        this.$.iframe.src = ui.urlForEmbeddedFlakinessDashboard(this.test.testName, this.test.step, this.tree);
       },
 
     });
   </script>
 </polymer-element>