Fix inobject slack tracking for both subclassing and non-subclassing cases.

test/cctest/cctest.gyp

 # Copyright 2012 the V8 project authors. 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
 #       copyright notice, this list of conditions and the following
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@@ skipping 124 matching lines @@
         'test-gc-tracer.cc',
         'test-global-handles.cc',
         'test-global-object.cc',
         'test-hashing.cc',
         'test-hashmap.cc',
         'test-heap.cc',
         'test-heap-profiler.cc',
         'test-hydrogen-types.cc',
         'test-identity-map.cc',
         'test-incremental-marking.cc',
+        'test-inobject-slack-tracking.cc',
         'test-list.cc',
         'test-liveedit.cc',
         'test-lockers.cc',
         'test-log.cc',
         'test-microtask-delivery.cc',
         'test-mark-compact.cc',
         'test-mementos.cc',
         'test-object-observe.cc',
         'test-parsing.cc',
         'test-platform.cc',
@@ skipping 197 matching lines @@
             '../../build/isolate.gypi',
           ],
           'sources': [
             'cctest.isolate',
           ],
         },
       ],
     }],
   ],
 }