Use Ash's ShelfWindowWatcher for app panel windows.

ash/common/shelf/shelf_window_watcher.h

 // Copyright 2013 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 ASH_COMMON_SHELF_SHELF_WINDOW_WATCHER_H_
 #define ASH_COMMON_SHELF_SHELF_WINDOW_WATCHER_H_
 
+#include <set>
+
 #include "ash/common/wm_activation_observer.h"
 #include "ash/common/wm_window_observer.h"
 #include "base/macros.h"
 #include "base/scoped_observer.h"
 #include "ui/display/display_observer.h"
 
 namespace ash {
 
 class ShelfModel;
 class WmWindow;
 
 // ShelfWindowWatcher creates and handles a ShelfItem for windows in the default
 // container that have a valid ShelfItemType property (e.g. the task manager
 // dialog or the OS settings window). It adds the ShelfItem when the window is
 // added to the default container and maintains it until the window is closed,
 // even if the window is transiently reparented (e.g. during a drag).

[James Cook, 2016/11/02 17:53:44]

nit: update comment to say it handles panels also

[msw, 2016/11/10 21:07:46]

Done.

 class ShelfWindowWatcher : public WmActivationObserver,
                            public display::DisplayObserver {
  public:
   explicit ShelfWindowWatcher(ShelfModel* model);
   ~ShelfWindowWatcher() override;
 
  private:
   // Observes for windows being added to a root window's default container.
   class ContainerWindowObserver : public WmWindowObserver {
    public:
@@ skipping 65 matching lines @@
                                uint32_t metrics) override;
 
   ShelfModel* model_;
 
   ContainerWindowObserver container_window_observer_;
   UserWindowObserver user_window_observer_;
 
   ScopedObserver<WmWindow, ContainerWindowObserver> observed_container_windows_;
   ScopedObserver<WmWindow, UserWindowObserver> observed_user_windows_;
 
+  // The set of windows with shelf items managed by this ShelfWindowWatcher.
+  std::set<WmWindow*> managed_user_windows_;

[James Cook, 2016/11/02 17:53:43]

I'm OK with this name, but what about user_windows_with_items_? I was a little
confused by the code to cleanup windows when they close.

[msw, 2016/11/10 21:07:46]

Done.

+
   DISALLOW_COPY_AND_ASSIGN(ShelfWindowWatcher);
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_SHELF_SHELF_WINDOW_WATCHER_H_