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

ash/common/shelf/shelf_model.cc

Index: ash/common/shelf/shelf_model.cc
diff --git a/ash/common/shelf/shelf_model.cc b/ash/common/shelf/shelf_model.cc
index a489fecb815ef5e1dda97ee02cd0144dbfed5e98..0d292a495bf30216455ea6fb7cf632afeba2b136 100644
--- a/ash/common/shelf/shelf_model.cc
+++ b/ash/common/shelf/shelf_model.cc
@@ -132,7 +132,7 @@ int ShelfModel::GetItemIndexForType(ShelfItemType type) {
   return -1;
 }
 
-ShelfItems::const_iterator ShelfModel::ItemByID(int id) const {
+ShelfItems::const_iterator ShelfModel::ItemByID(ShelfID id) const {
   for (ShelfItems::const_iterator i = items_.begin(); i != items_.end(); ++i) {
     if (i->id == id)
       return i;