Reduce imprecision of incremental marking.

src/mark-compact.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 101 matching lines @@
     ASSERT(IsGrey(markbit));
     markbit.Next().Clear();
     ASSERT(IsBlack(markbit));
   }
 
   static inline void BlackToGrey(HeapObject* obj) {
     ASSERT(obj->Size() >= 2 * kPointerSize);
     BlackToGrey(MarkBitFrom(obj));
   }
 
+  static inline void AnyToGrey(MarkBit markbit) {
+    markbit.Set();
+    markbit.Next().Set();
+    ASSERT(IsGrey(markbit));
+  }
+
   // Returns true if the the object whose mark is transferred is marked black.
   bool TransferMark(Address old_start, Address new_start);
 
 #ifdef DEBUG
   enum ObjectColor {
     BLACK_OBJECT,
     WHITE_OBJECT,
     GREY_OBJECT,
     IMPOSSIBLE_COLOR
   };
@@ skipping 623 matching lines @@
 
   List<Page*> evacuation_candidates_;
 
   friend class Heap;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_MARK_COMPACT_H_