Revert of 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 8c79ac54cbebde1de7c8146d42116ce31c5e3375..0e4d75d0ccdcbfa8cfe6e07dfb27c4b3ab52ef06 100644
--- a/extensions/test/extension_test_notification_observer.cc
+++ b/extensions/test/extension_test_notification_observer.cc
@@ -141,9 +141,6 @@
 void ExtensionTestNotificationObserver::Wait() {
   observer_->Wait();
 
-  // TODO(https://crbug.com/695073): Find out why tests fail without it.
-  content::RunAllPendingInMessageLoop();
-
   registrar_.RemoveAll();
   observer_.reset();
 }
@@ -190,15 +187,16 @@
     return;
   condition_ = condition;
 
-  base::RunLoop run_loop;
-  quit_closure_ = run_loop.QuitClosure();
+  scoped_refptr<content::MessageLoopRunner> runner(
+      new content::MessageLoopRunner);
+  quit_closure_ = runner->QuitClosure();
 
   std::unique_ptr<base::CallbackList<void()>::Subscription> subscription;
   if (notification_set) {
     subscription = notification_set->callback_list().Add(base::Bind(
         &ExtensionTestNotificationObserver::MaybeQuit, base::Unretained(this)));
   }
-  run_loop.Run();
+  runner->Run();
 
   condition_.Reset();
   quit_closure_.Reset();