Add a floating close button for arc custom notification

ui/arc/notification/arc_custom_notification_item.h

Index: ui/arc/notification/arc_custom_notification_item.h
diff --git a/ui/arc/notification/arc_custom_notification_item.h b/ui/arc/notification/arc_custom_notification_item.h
index 54c35ab0eb32038b1f75ccfb1d7fd36df10faa5e..5bc55b916ef33e2da1ff3195a7cd516f55011288 100644
--- a/ui/arc/notification/arc_custom_notification_item.h
+++ b/ui/arc/notification/arc_custom_notification_item.h
@@ -6,6 +6,7 @@
 #define UI_ARC_NOTIFICATION_ARC_CUSTOM_NOTIFICATION_ITEM_H_
 
 #include "base/macros.h"
+#include "base/observer_list.h"
 #include "ui/arc/notification/arc_notification_item.h"
 #include "ui/arc/notification/arc_notification_surface_manager.h"
 
@@ -15,6 +16,18 @@ class ArcCustomNotificationItem
     : public ArcNotificationItem,
       public ArcNotificationSurfaceManager::Observer {
  public:
+  class Observer {
+   public:
+    // Invoked when the notification data for this item has changed.
+    virtual void OnItemDestroying() = 0;
+
+    // Invoked when the pinned stated is changed.
+    virtual void OnItemPinnedChanged() = 0;
+
+   protected:
+    virtual ~Observer() = default;
+  };
+
   ArcCustomNotificationItem(ArcNotificationManager* manager,
                             message_center::MessageCenter* message_center,
                             const std::string& notification_key,
@@ -24,11 +37,21 @@ class ArcCustomNotificationItem
   void UpdateWithArcNotificationData(
       const mojom::ArcNotificationData& data) override;
 
+  void CloseFromCloseButton();
+
+  void AddObserver(Observer* observer);
+  void RemoveObserver(Observer* observer);
+
+  bool pinned() const { return pinned_; }
+
  private:
   // ArcNotificationSurfaceManager::Observer:
   void OnNotificationSurfaceAdded(exo::NotificationSurface* surface) override;
   void OnNotificationSurfaceRemoved(exo::NotificationSurface* surface) override;
 
+  bool pinned_ = false;
+  base::ObserverList<Observer> observers_;
+
   DISALLOW_COPY_AND_ASSIGN(ArcCustomNotificationItem);
 };