Fixes a DCHECK in tools_menu_button_observer_bridge during tests

components/reading_list/core/reading_list_model_unittest.cc

Index: components/reading_list/core/reading_list_model_unittest.cc
diff --git a/components/reading_list/core/reading_list_model_unittest.cc b/components/reading_list/core/reading_list_model_unittest.cc
index a2b2b57fe4634fde0e989f6a97ed6bafd7cb0e2b..c79a63578dc12643dfaa0e2f6b918293fc76dbd3 100644
--- a/components/reading_list/core/reading_list_model_unittest.cc
+++ b/components/reading_list/core/reading_list_model_unittest.cc
@@ -314,6 +314,9 @@ TEST_F(ReadingListModelTest, EmptyLoaded) {
   EXPECT_EQ(0ul, ReadSize());
   model_->Shutdown();
   EXPECT_FALSE(model_->loaded());
+  // Shutdown() does not delete the model observer. Verify that deleting the
+  // model will delete the model observer.
+  model_.reset();
   AssertObserverCount(1, 0, 0, 1, 0, 0, 0, 0, 0);
 }