Remove usage of FOR_EACH_OBSERVER macro in ui/message_center

ui/message_center/notification_blocker.cc

Index: ui/message_center/notification_blocker.cc
diff --git a/ui/message_center/notification_blocker.cc b/ui/message_center/notification_blocker.cc
index 9ad1656b64e224e1959fc4f63eb45f9aae5e9c56..80353e8bdd9be238889d5258a61de32e70489519 100644
--- a/ui/message_center/notification_blocker.cc
+++ b/ui/message_center/notification_blocker.cc
@@ -34,8 +34,8 @@ bool NotificationBlocker::ShouldShowNotification(
 }
 
 void NotificationBlocker::NotifyBlockingStateChanged() {
-  FOR_EACH_OBSERVER(
-      NotificationBlocker::Observer, observers_, OnBlockingStateChanged(this));
+  for (auto& observer : observers_)
+    observer.OnBlockingStateChanged(this);
 }
 
 }  // namespace message_center