Fixes a DCHECK in tools_menu_button_observer_bridge during tests

components/reading_list/ios/reading_list_model_bridge_observer.mm

Index: components/reading_list/ios/reading_list_model_bridge_observer.mm
diff --git a/components/reading_list/ios/reading_list_model_bridge_observer.mm b/components/reading_list/ios/reading_list_model_bridge_observer.mm
index f295c7fdf6302b6d9d2c059a81868f2f978fa638..a54a1a471f14e790b8a5a887f66c1687882b9534 100644
--- a/components/reading_list/ios/reading_list_model_bridge_observer.mm
+++ b/components/reading_list/ios/reading_list_model_bridge_observer.mm
@@ -26,6 +26,14 @@ void ReadingListModelBridge::ReadingListModelLoaded(
   [observer_ readingListModelLoaded:model];
 }
 
+void ReadingListModelBridge::ReadingListModelBeingShutdown(
+    const ReadingListModel* model) {
+  if ([observer_
+          respondsToSelector:@selector(readingListModelBeingShutdown:)]) {
+    [observer_ readingListModelBeingShutdown:model];
+  }
+}
+
 void ReadingListModelBridge::ReadingListModelBeingDeleted(
     const ReadingListModel* model) {
   if ([observer_ respondsToSelector:@selector(readingListModelBeingDeleted:)]) {