Use idle task to perform incremental marking steps.

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
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 115 matching lines @@
         'test-func-name-inference.cc',
         '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-list.cc',
         'test-liveedit.cc',
         'test-lockers.cc',
         'test-log.cc',
         'test-microtask-delivery.cc',
         'test-mark-compact.cc',
         'test-mementos.cc',
         'test-migrations.cc',
         'test-object-observe.cc',
         'test-parsing.cc',
@@ skipping 173 matching lines @@
             '../../tools/js2c.py',
             '<@(_outputs)',
             'TEST',  # type
             '<@(file_list)',
           ],
         }
       ],
     },
   ],
 }