Extend notifications.mojom for expandable notifications.

ui/message_center/notification.h

Index: ui/message_center/notification.h
diff --git a/ui/message_center/notification.h b/ui/message_center/notification.h
index 8b66c656c927bd5f5fecb5de701dff502e2ab4d8..013217321af304f605447f2acd1a6318bc0f6c87 100644
--- a/ui/message_center/notification.h
+++ b/ui/message_center/notification.h
@@ -53,6 +53,8 @@ struct MESSAGE_CENTER_EXPORT ButtonInfo {
   ButtonInfo& operator=(const ButtonInfo& other);
 };
 
+enum class NotificationExpandState { FIXED_SIZE, COLLAPSED, EXPANDED };
+
 class MESSAGE_CENTER_EXPORT RichNotificationData {
  public:
   RichNotificationData();
@@ -74,6 +76,8 @@ class MESSAGE_CENTER_EXPORT RichNotificationData {
   // Flag if the notification is pinned. If true, the notification is pinned
   // and user can't remove it.
   bool pinned;
+  // For the expand state of the notification
+  NotificationExpandState expand_state;
 #endif  // defined(OS_CHROMEOS)
   std::vector<int> vibration_pattern;
   bool renotify;