Fix crash from DescendantInvalidationSet::show().

Source/core/css/invalidation/DescendantInvalidationSet.cpp

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 247 matching lines @@
         value->endArray();
     }
 
     value->endDictionary();
 }
 
 #ifndef NDEBUG
 void DescendantInvalidationSet::show() const
 {
     RefPtr<TracedValue> value = TracedValue::create();
+    value->beginArray("DescendantInvalidationSet");
     toTracedValue(value.get());
+    value->endArray();
     fprintf(stderr, "%s\n", value->asTraceFormat().ascii().data());
 }
 #endif // NDEBUG
 
 } // namespace blink