Use Ash's ShelfWindowWatcher for app panel windows.

ash/common/shelf/shelf_window_watcher.h

Index: ash/common/shelf/shelf_window_watcher.h
diff --git a/ash/common/shelf/shelf_window_watcher.h b/ash/common/shelf/shelf_window_watcher.h
index 45565e8f4b67f375d0109445f62bf5a321277683..d2dccf53dd32b081f841edf6b8303175bc4e2137 100644
--- a/ash/common/shelf/shelf_window_watcher.h
+++ b/ash/common/shelf/shelf_window_watcher.h
@@ -5,6 +5,8 @@
 #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"
@@ -106,6 +108,9 @@ class ShelfWindowWatcher : public WmActivationObserver,
   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);
 };