Use Ash's ShelfWindowWatcher for app panel windows.

ash/common/shelf/shelf_model.cc

Index: ash/common/shelf/shelf_model.cc
diff --git a/ash/common/shelf/shelf_model.cc b/ash/common/shelf/shelf_model.cc
index cd177c231ff52af580e0e5c5c9be7eedbc11747c..13e0b922a41427e25125abc9adb8f6d9e30ae7b4 100644
--- a/ash/common/shelf/shelf_model.cc
+++ b/ash/common/shelf/shelf_model.cc
@@ -71,9 +71,6 @@ int ShelfModel::AddAt(int index, const ShelfItem& item) {
 
 void ShelfModel::RemoveItemAt(int index) {
   DCHECK(index >= 0 && index < item_count());
-  // The app list and browser shortcut can't be removed.
-  DCHECK(items_[index].type != TYPE_APP_LIST &&
-         items_[index].type != TYPE_BROWSER_SHORTCUT);
   ShelfID id = items_[index].id;
   items_.erase(items_.begin() + index);
   RemoveShelfItemDelegate(id);