mash: Support ShelfModel access in Chrome.

ash/shelf/shelf_model.cc

Index: ash/shelf/shelf_model.cc
diff --git a/ash/shelf/shelf_model.cc b/ash/shelf/shelf_model.cc
index e5ddd25680b09483ebeb2074dd498ffeed4b7f89..85262e384df30b90eecb560bb06aedb520296531 100644
--- a/ash/shelf/shelf_model.cc
+++ b/ash/shelf/shelf_model.cc
@@ -106,8 +106,8 @@ int ShelfModel::Add(const ShelfItem& item) {
 
 int ShelfModel::AddAt(int index, const ShelfItem& item) {
   // Items should have unique non-empty ids to avoid undefined model behavior.
-  DCHECK(!item.id.IsNull());
-  DCHECK_EQ(ItemIndexByID(item.id), -1);
+  DCHECK(!item.id.IsNull()) << " The id is null.";
+  DCHECK_EQ(ItemIndexByID(item.id), -1) << " The id is not unique: " << item.id;
   index = ValidateInsertionIndex(item.type, index);
   items_.insert(items_.begin() + index, item);
   for (auto& observer : observers_)
@@ -210,6 +210,10 @@ void ShelfModel::SetShelfItemDelegate(
     item_delegate->set_shelf_id(shelf_id);
   // This assignment replaces any ShelfItemDelegate already registered for |id|.
   id_to_item_delegate_map_[shelf_id] = std::move(item_delegate);
+  for (auto& observer : observers_) {
+    observer.ShelfItemDelegateChanged(shelf_id,
+                                      id_to_item_delegate_map_[shelf_id].get());
+  }
 }
 
 ShelfItemDelegate* ShelfModel::GetShelfItemDelegate(const ShelfID& shelf_id) {