Clear mark bits while sweeping and not explicitly.

src/incremental-marking.h

 // Copyright 2011 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 184 matching lines @@
   void ResetStepCounters() {
     steps_count_ = 0;
     steps_took_ = 0;
     longest_step_ = 0.0;
     steps_count_since_last_gc_ = 0;
     steps_took_since_last_gc_ = 0;
     bytes_rescanned_ = 0;
     allocation_marking_factor_ = kInitialAllocationMarkingFactor;
   }
 
-  static void ClearMarkbits(PagedSpace* space);
-  static void ClearMarkbits(NewSpace* space);
-  void ClearMarkbits();
-
-#ifdef DEBUG
-  void VerifyMarkbitsAreClean();
-  static void VerifyMarkbitsAreClean(PagedSpace* space);
-  static void VerifyMarkbitsAreClean(NewSpace* space);
-#endif
-
   void StartMarking();
 
-  void DeactivateIncrementalWriteBarrierForSpace(PagedSpace* space);
-  void DeactivateIncrementalWriteBarrierForSpace(NewSpace* space);
+  static void ActivateIncrementalWriteBarrier(PagedSpace* space);
+  static void ActivateIncrementalWriteBarrier(NewSpace* space);
+  void ActivateIncrementalWriteBarrier();
+
+  static void DeactivateIncrementalWriteBarrierForSpace(PagedSpace* space);
+  static void DeactivateIncrementalWriteBarrierForSpace(NewSpace* space);
   void DeactivateIncrementalWriteBarrier();
 
   static void SetOldSpacePageFlags(MemoryChunk* chunk, bool is_marking);
   static void SetNewSpacePageFlags(NewSpacePage* chunk, bool is_marking);
 
   void EnsureMarkingDequeIsCommitted();
 
   Heap* heap_;
 
   State state_;
@@ skipping 11 matching lines @@
   bool should_hurry_;
   int allocation_marking_factor_;
   intptr_t allocated_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(IncrementalMarking);
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_INCREMENTAL_MARKING_H_