Remove FOR_EACH_OBSERVER from arc/ since it's being deprecated

ui/arc/notification/arc_custom_notification_item.cc

Index: ui/arc/notification/arc_custom_notification_item.cc
diff --git a/ui/arc/notification/arc_custom_notification_item.cc b/ui/arc/notification/arc_custom_notification_item.cc
index 24fcbb49476b7d71b9a3ba1e662953f6648b271f..f3998a5d5fb566995f5db5bfa03ac42f5a4d270e 100644
--- a/ui/arc/notification/arc_custom_notification_item.cc
+++ b/ui/arc/notification/arc_custom_notification_item.cc
@@ -49,7 +49,8 @@ ArcCustomNotificationItem::ArcCustomNotificationItem(
 }
 
 ArcCustomNotificationItem::~ArcCustomNotificationItem() {
-  FOR_EACH_OBSERVER(Observer, observers_, OnItemDestroying());
+  for (auto& observer : observers_)
+    observer.OnItemDestroying();
 }
 
 void ArcCustomNotificationItem::UpdateWithArcNotificationData(
@@ -93,7 +94,8 @@ void ArcCustomNotificationItem::UpdateWithArcNotificationData(
         *data->snapshot_image, data->snapshot_image_scale));
   }
 
-  FOR_EACH_OBSERVER(Observer, observers_, OnItemUpdated());
+  for (auto& observer : observers_)
+    observer.OnItemUpdated();
 
   AddToMessageCenter();
 }