Fix flaky WindowsApiSetIcon test.

chrome/test/data/extensions/platform_apps/windows_api_set_icon/background.js

Index: chrome/test/data/extensions/platform_apps/windows_api_set_icon/background.js
diff --git a/chrome/test/data/extensions/platform_apps/windows_api_set_icon/background.js b/chrome/test/data/extensions/platform_apps/windows_api_set_icon/background.js
index 85029bd662d667f23b01952fc6508840e416250b..5c8e1f869a24940b9021f441cce354c73e9831b9 100644
--- a/chrome/test/data/extensions/platform_apps/windows_api_set_icon/background.js
+++ b/chrome/test/data/extensions/platform_apps/windows_api_set_icon/background.js
@@ -3,9 +3,9 @@
 // found in the LICENSE file.
 
 chrome.app.runtime.onLaunched.addListener(function() {
-  chrome.test.sendMessage("Launched");
   chrome.app.window.create('test.html', {}, function(win) {
-    win.setIcon("icon.png");
-    chrome.test.sendMessage('IconSet');
+    chrome.test.sendMessage('ready', function() {
+      win.setIcon("icon.png");
+    });
   });
 });