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

ash/common/shelf/app_list_shelf_item_delegate.cc

 // Copyright 2013 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.
 
 #include "ash/common/shelf/app_list_shelf_item_delegate.h"
 
 #include "ash/common/shelf/shelf_model.h"
 #include "ash/shell.h"
 #include "ash/strings/grit/ash_strings.h"
 #include "base/memory/ptr_util.h"
 #include "ui/app_list/app_list_switches.h"
 #include "ui/base/l10n/l10n_util.h"
 
 namespace ash {
 
 // static
 void AppListShelfItemDelegate::CreateAppListItemAndDelegate(ShelfModel* model) {
   // Add the app list item to the shelf model.
   ShelfItem item;
   item.type = TYPE_APP_LIST;
   item.title = l10n_util::GetStringUTF16(IDS_ASH_SHELF_APP_LIST_LAUNCHER_TITLE);
   int index = model->Add(item);
   DCHECK_GE(index, 0);
 
   // Create an AppListShelfItemDelegate for that item.
   ShelfID id = model->items()[index].id;
-  DCHECK_GE(id, 0);
+  DCHECK_NE(id, kInvalidShelfID);
   model->SetShelfItemDelegate(id, base::MakeUnique<AppListShelfItemDelegate>());
 }
 
 AppListShelfItemDelegate::AppListShelfItemDelegate() {}
 
 AppListShelfItemDelegate::~AppListShelfItemDelegate() {}
 
 void AppListShelfItemDelegate::ItemSelected(
     std::unique_ptr<ui::Event> event,
     int64_t display_id,
     ShelfLaunchSource source,
     const ItemSelectedCallback& callback) {
   Shell::Get()->ToggleAppList();
   callback.Run(SHELF_ACTION_APP_LIST_SHOWN, base::nullopt);
 }
 
 void AppListShelfItemDelegate::ExecuteCommand(uint32_t command_id,
                                               int32_t event_flags) {
   // This delegate does not support showing an application menu.
   NOTIMPLEMENTED();
 }
 
 void AppListShelfItemDelegate::Close() {}
 
 }  // namespace ash