Add a floating close button for arc custom notification

ui/arc/notification/arc_custom_notification_item.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_ARC_NOTIFICATION_ARC_CUSTOM_NOTIFICATION_ITEM_H_
 #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"
 
 namespace arc {
 
 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,
                             const AccountId& profile_id);
   ~ArcCustomNotificationItem() override;
 
   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);
 };
 
 }  // namespace arc
 
 #endif  // UI_ARC_NOTIFICATION_ARC_CUSTOM_NOTIFICATION_ITEM_H_