Fix mash shelf id mapping and removal.

ash/mus/shelf_delegate_mus.h

Index: ash/mus/shelf_delegate_mus.h
diff --git a/ash/mus/shelf_delegate_mus.h b/ash/mus/shelf_delegate_mus.h
index 7b79c691c5ab2333b9179f44b94b68d1dad0252a..54715ec901e1f5a51aed7a5fd303176ed89313c7 100644
--- a/ash/mus/shelf_delegate_mus.h
+++ b/ash/mus/shelf_delegate_mus.h
@@ -75,6 +75,7 @@ class ShelfDelegateMus : public ShelfDelegate,
   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);
 };