Fix crash from DescendantInvalidationSet::show().

Source/core/css/invalidation/DescendantInvalidationSetTest.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "core/css/invalidation/DescendantInvalidationSet.h"
 
 #include <gtest/gtest.h>
 
 using namespace blink;
@@ skipping 43 matching lines @@
 
     set1->setWholeSubtreeInvalid();
     set2->addAttribute("a");
 
     set1->combine(*set2);
 
     ASSERT_TRUE(set1->wholeSubtreeInvalid());
     ASSERT_TRUE(set1->isEmpty());
 }
 
+#ifndef NDEBUG
+TEST(DescendantInvalidationSetTest, ShowDebug)
+{
+    RefPtrWillBeRawPtr<DescendantInvalidationSet> set = DescendantInvalidationSet::create();
+    set->show();
+}
+#endif // NDEBUG
+
 } // namespace