Use Ash's ShelfWindowWatcher for app panel windows.

ash/common/shelf/shelf_window_watcher_item_delegate.h

Index: ash/common/shelf/shelf_window_watcher_item_delegate.h
diff --git a/ash/common/shelf/shelf_window_watcher_item_delegate.h b/ash/common/shelf/shelf_window_watcher_item_delegate.h
index 895e56a7fe36e446836bfbd15ad18d3782746b1c..343c201a3cf151e7dc6f93637053799f49a25b8a 100644
--- a/ash/common/shelf/shelf_window_watcher_item_delegate.h
+++ b/ash/common/shelf/shelf_window_watcher_item_delegate.h
@@ -6,6 +6,7 @@
 #define ASH_COMMON_SHELF_SHELF_WINDOW_WATCHER_ITEM_DELEGATE_H_
 
 #include "ash/common/shelf/shelf_item_delegate.h"
+#include "ash/common/shelf/shelf_item_types.h"
 #include "base/macros.h"
 
 namespace ash {
@@ -15,7 +16,7 @@ class WmWindow;
 // ShelfItemDelegate for the items created by ShelfWindowWatcher.
 class ShelfWindowWatcherItemDelegate : public ShelfItemDelegate {
  public:
-  explicit ShelfWindowWatcherItemDelegate(WmWindow* window);
+  ShelfWindowWatcherItemDelegate(ShelfID id, WmWindow* window);
   ~ShelfWindowWatcherItemDelegate() override;
 
  private:
@@ -29,6 +30,7 @@ class ShelfWindowWatcherItemDelegate : public ShelfItemDelegate {
   bool ShouldShowTooltip() override;
   void Close() override;
 
+  ShelfID id_;
   // The window associated with this item. Not owned.
   WmWindow* window_;