Enable incremental marking when --expose-gc is turned on.

src/heap.cc

 // 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 5601 matching lines @@
       AdvanceIdleIncrementalMarking(step_size);
     }
 
     // After context disposal there is likely a lot of garbage remaining, reset
     // the idle notification counters in order to trigger more incremental GCs
     // on subsequent idle notifications.
     StartIdleRound();
     return false;
   }
 
-  if (!FLAG_incremental_marking || FLAG_expose_gc || Serializer::enabled()) {
+  if (!FLAG_incremental_marking || Serializer::enabled()) {
     return IdleGlobalGC();
   }
 
   // By doing small chunks of GC work in each IdleNotification,
   // perform a round of incremental GCs and after that wait until
   // the mutator creates enough garbage to justify a new round.
   // An incremental GC progresses as follows:
   // 1. many incremental marking steps,
   // 2. one old space mark-sweep-compact,
   // 3. many lazy sweep steps.
@@ skipping 2104 matching lines @@
       static_cast<int>(object_sizes_last_time_[index]));
   CODE_AGE_LIST_COMPLETE(ADJUST_LAST_TIME_OBJECT_COUNT)
 #undef ADJUST_LAST_TIME_OBJECT_COUNT
 
   OS::MemCopy(object_counts_last_time_, object_counts_, sizeof(object_counts_));
   OS::MemCopy(object_sizes_last_time_, object_sizes_, sizeof(object_sizes_));
   ClearObjectStats();
 }
 
 } }  // namespace v8::internal