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

ash/common/shelf/app_list_shelf_item_delegate.cc

Index: ash/common/shelf/app_list_shelf_item_delegate.cc
diff --git a/ash/common/shelf/app_list_shelf_item_delegate.cc b/ash/common/shelf/app_list_shelf_item_delegate.cc
index 42bf94d7f0f489b616705dc62d64e1ba31c42e1f..c3da04402799a8d487dc1a8e8f9740e7779ea19d 100644
--- a/ash/common/shelf/app_list_shelf_item_delegate.cc
+++ b/ash/common/shelf/app_list_shelf_item_delegate.cc
@@ -24,7 +24,7 @@ void AppListShelfItemDelegate::CreateAppListItemAndDelegate(ShelfModel* model) {
 
   // 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>());
 }