Fix mash shelf id mapping and removal.

ash/mus/shelf_delegate_mus.h

 // Copyright 2016 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_MUS_SHELF_DELEGATE_MUS_H_
 #define ASH_MUS_SHELF_DELEGATE_MUS_H_
 
 #include <map>
 
 #include "ash/shelf/shelf_delegate.h"
@@ skipping 57 matching lines @@
   ShelfModel* model_;
 
   mojo::AssociatedInterfacePtrSet<mash::shelf::mojom::ShelfObserver> observers_;
 
   mash::wm::mojom::ShelfLayoutPtr shelf_layout_;
   mash::wm::mojom::UserWindowControllerPtr user_window_controller_;
   mojo::Binding<mash::wm::mojom::UserWindowObserver> binding_;
   std::map<uint32_t, ShelfID> window_id_to_shelf_id_;
 
   std::map<std::string, ShelfID> app_id_to_shelf_id_;
+  std::map<ShelfID, std::string> shelf_id_to_app_id_;
 
   DISALLOW_COPY_AND_ASSIGN(ShelfDelegateMus);
 };
 
 }  // namespace sysui
 }  // namespace ash
 
 #endif  // ASH_MUS_SHELF_DELEGATE_MUS_H_