mash: Fix ShelfItem mojo struct; add enums and traits.

ash/public/cpp/shelf_types.h

 // Copyright (c) 2012 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_PUBLIC_CPP_SHELF_TYPES_H_
 #define ASH_PUBLIC_CPP_SHELF_TYPES_H_
 
 #include <cstdint>
 
 #include "ash/public/cpp/ash_public_export.h"
@@ skipping 71 matching lines @@
   // An existing inactive window was activated.
   SHELF_ACTION_WINDOW_ACTIVATED,
 
   // The currently active window was minimized.
   SHELF_ACTION_WINDOW_MINIMIZED,
 
   // The app list launcher menu was shown.
   SHELF_ACTION_APP_LIST_SHOWN,
 };
 
-typedef int ShelfID;
-const int kInvalidShelfID = 0;
+// Shelf IDs are sequential values assigned to shelf items by ShelfModel for
+// runtime use; they are not persisted across restarts of the ash environment.
+using ShelfID = uint32_t;
+const ShelfID kInvalidShelfID = 0u;
 
 // The type of a shelf item.
 enum ShelfItemType {
   // Represents a running app panel.
   TYPE_APP_PANEL,
 
   // Represents a pinned shortcut to an app, the app may be running or not.
   TYPE_PINNED_APP,
 
   // Toggles visiblity of the app list.
@@ skipping 26 matching lines @@
   STATUS_RUNNING,
   // An active shelf item that has focus.
   STATUS_ACTIVE,
   // A shelf item that needs user's attention.
   STATUS_ATTENTION,
 };
 
 }  // namespace ash
 
 #endif  // ASH_PUBLIC_CPP_SHELF_TYPES_H_