Use Ash's ShelfWindowWatcher for app panel windows.

ash/shell/panel_window.cc

Index: ash/shell/panel_window.cc
diff --git a/ash/shell/panel_window.cc b/ash/shell/panel_window.cc
index 168649f032fbe31eec1674af5ac0fc88bfa837c8..f9150b8203b97a957540bbb63585c382939c6d44 100644
--- a/ash/shell/panel_window.cc
+++ b/ash/shell/panel_window.cc
@@ -7,9 +7,9 @@
 #include "ash/common/wm/panels/panel_frame_view.h"
 #include "ash/screen_util.h"
 #include "ash/shell.h"
+#include "ash/wm/window_properties.h"
 #include "base/strings/utf_string_conversions.h"
 #include "ui/aura/window.h"
-#include "ui/aura/window_event_dispatcher.h"
 #include "ui/gfx/canvas.h"
 #include "ui/views/widget/widget.h"
 
@@ -49,6 +49,8 @@ views::Widget* PanelWindow::CreateWidget() {
 
   widget->Init(params());
   widget->GetNativeView()->SetName(name_);
+  widget->GetNativeWindow()->SetProperty<int>(kShelfItemTypeKey,
+                                              TYPE_APP_PANEL);
   widget->Show();
 
   return widget;