Simplify ExtensionInstallChecker into a single-use class

extensions/test/extension_test_notification_observer.cc

Index: extensions/test/extension_test_notification_observer.cc
diff --git a/extensions/test/extension_test_notification_observer.cc b/extensions/test/extension_test_notification_observer.cc
index 0e4d75d0ccdcbfa8cfe6e07dfb27c4b3ab52ef06..7d6db8079ff890d6bee9122c62423b9ae8638e66 100644
--- a/extensions/test/extension_test_notification_observer.cc
+++ b/extensions/test/extension_test_notification_observer.cc
@@ -127,7 +127,8 @@ bool ExtensionTestNotificationObserver::WaitForExtensionCrash(
 bool ExtensionTestNotificationObserver::WaitForCrxInstallerDone() {
   int before = crx_installers_done_observed_;
   WaitForNotification(NOTIFICATION_CRX_INSTALLER_DONE);
-  return crx_installers_done_observed_ == before + 1;
+  return crx_installers_done_observed_ == before + 1 &&
+         !last_loaded_extension_id_.empty();
 }
 
 void ExtensionTestNotificationObserver::Watch(