Fix race-condition in ct-embedded-flakiness-dashboard-tests.

Tools/GardeningServer/ui/ct-embedded-flakiness-dashboard-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.
 -->
 
 <link rel="import" href="ct-embedded-flakiness-dashboard.html">
 
 <script>
 (function() {
@@ skipping 18 matching lines @@
       reset();
       start();
     });
   } catch (e) {
     reset();
   }
 });
 
 asyncTest("heightChanged", 1, function() {
   var embeddedFlakinessDashboard = document.createElement('ct-embedded-flakiness-dashboard');
-  window.postMessage({command: 'heightChanged', height: "100"}, '*')
 
-  requestAnimationFrame(function() {
+  function finishTest() {
+    window.removeEventListener('message', finishTest);
     var iframe = embeddedFlakinessDashboard.shadowRoot.querySelector('#iframe');
     equal(iframe.style.height, "100px");
     start();
+  }
+  window.addEventListener('message', finishTest);
+
+  requestAnimationFrame(function() {
+    window.postMessage({command: 'heightChanged', height: "100"}, '*');
   });
 });
 
 })()
 </script>