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

Tools/GardeningServer/ui/ct-embedded-flakiness-dashboard-tests.html

Index: Tools/GardeningServer/ui/ct-embedded-flakiness-dashboard-tests.html
diff --git a/Tools/GardeningServer/ui/ct-embedded-flakiness-dashboard-tests.html b/Tools/GardeningServer/ui/ct-embedded-flakiness-dashboard-tests.html
index 6749a205b1b017f901b350004d9ac1baa50479e8..8aa0f33268bb2cae4fc890a983b270a035343ad2 100644
--- a/Tools/GardeningServer/ui/ct-embedded-flakiness-dashboard-tests.html
+++ b/Tools/GardeningServer/ui/ct-embedded-flakiness-dashboard-tests.html
@@ -36,12 +36,17 @@ asyncTest("url", 1, function() {
 
 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"}, '*');
   });
 });