Fix crash from DescendantInvalidationSet::show().

Source/core/css/invalidation/DescendantInvalidationSetTest.cpp

Index: Source/core/css/invalidation/DescendantInvalidationSetTest.cpp
diff --git a/Source/core/css/invalidation/DescendantInvalidationSetTest.cpp b/Source/core/css/invalidation/DescendantInvalidationSetTest.cpp
index f2735ec1e3dd17c73549e83eebf80ff175fcb22a..e201215175de8c0d520426c11f441128ea74a3c4 100644
--- a/Source/core/css/invalidation/DescendantInvalidationSetTest.cpp
+++ b/Source/core/css/invalidation/DescendantInvalidationSetTest.cpp
@@ -61,4 +61,12 @@ TEST(DescendantInvalidationSetTest, SubtreeInvalid_Combine_2)
     ASSERT_TRUE(set1->isEmpty());
 }
 
+#ifndef NDEBUG
+TEST(DescendantInvalidationSetTest, ShowDebug)
+{
+    RefPtrWillBeRawPtr<DescendantInvalidationSet> set = DescendantInvalidationSet::create();
+    set->show();
+}
+#endif // NDEBUG
+
 } // namespace