Fix flaky WindowsApiSetIcon test.

chrome/browser/extensions/api/app_window/app_window_apitest.cc

Index: chrome/browser/extensions/api/app_window/app_window_apitest.cc
diff --git a/chrome/browser/extensions/api/app_window/app_window_apitest.cc b/chrome/browser/extensions/api/app_window/app_window_apitest.cc
index 5b20a4226b85d87f31c8734628324d0d1f95ba6e..2420628ad7a3ddee05b023b3ca3da34554dea3d7 100644
--- a/chrome/browser/extensions/api/app_window/app_window_apitest.cc
+++ b/chrome/browser/extensions/api/app_window/app_window_apitest.cc
@@ -51,8 +51,13 @@ namespace extensions {
 IN_PROC_BROWSER_TEST_F(ExperimentalPlatformAppBrowserTest, WindowsApiSetIcon) {
   scoped_ptr<TestAppWindowRegistryObserver> test_observer(
       new TestAppWindowRegistryObserver(browser()->profile()));
-  LoadAndLaunchPlatformApp("windows_api_set_icon", "IconSet");
+  ExtensionTestMessageListener listener("ready", true);
+
+  // Launch the app and wait for it to be ready.
+  LoadAndLaunchPlatformApp("windows_api_set_icon", &listener);
   EXPECT_EQ(0, test_observer->icon_updates());
+  listener.Reply("");
+
   // Now wait until the WebContent has decoded the icon and chrome has
   // processed it. This needs to be in a loop since the renderer runs in a
   // different process.