Fix Mac App Launcher observer lifetime on AppListItemList

ui/app_list/app_list_item_list.cc

Index: ui/app_list/app_list_item_list.cc
diff --git a/ui/app_list/app_list_item_list.cc b/ui/app_list/app_list_item_list.cc
index c8d96e2aec1a7c35ffd54ebf6079fbdd45f21b98..e7c935081d1322b6f6391c5f4bf8ea069c68505b 100644
--- a/ui/app_list/app_list_item_list.cc
+++ b/ui/app_list/app_list_item_list.cc
@@ -19,6 +19,7 @@ void AppListItemList::AddObserver(AppListItemListObserver* observer) {
 }
 
 void AppListItemList::RemoveObserver(AppListItemListObserver* observer) {
+  DCHECK(observers_.HasObserver(observer));
   observers_.RemoveObserver(observer);
 }