Switch WindowedNotificationObserver to use base::RunLoop.

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..a18e75fc4914fef6ae2838f5de18fed360b3aa9e 100644
--- a/extensions/test/extension_test_notification_observer.cc
+++ b/extensions/test/extension_test_notification_observer.cc
@@ -140,6 +140,8 @@ void ExtensionTestNotificationObserver::Watch(
 
 void ExtensionTestNotificationObserver::Wait() {
   observer_->Wait();
+  // TODO(...): Find out why tests fail without it.
+  content::RunAllPendingInMessageLoop();
 
   registrar_.RemoveAll();
   observer_.reset();