Extend notifications.mojom for expandable notifications.

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 f44884dd5c039ad80cf19ab073be0df5c6b3e581..e0b0417d9f757b67533dc333128f9b4e2ba3c262 100644
--- a/ui/arc/notification/arc_custom_notification_item.cc
+++ b/ui/arc/notification/arc_custom_notification_item.cc
@@ -101,6 +101,7 @@ void ArcCustomNotificationItem::UpdateWithArcNotificationData(
   SetNotification(std::move(notification));
 
   pinned_ = rich_data.pinned;
+  expand_state_ = data->expand_state;
 
   if (!data->snapshot_image || data->snapshot_image->isNull()) {
     snapshot_ = gfx::ImageSkia();