MarkBit cleanup: They have to be accessed through Marking accessors.

src/heap/incremental-marking.cc

Index: src/heap/incremental-marking.cc
diff --git a/src/heap/incremental-marking.cc b/src/heap/incremental-marking.cc
index e01c50b849a0bb99053fa4f0b57f635280ae755c..dfcd68a3d34326388682265bf937aadf340991c3 100644
--- a/src/heap/incremental-marking.cc
+++ b/src/heap/incremental-marking.cc
@@ -141,7 +141,6 @@ static inline void MarkBlackOrKeepBlack(HeapObject* heap_object,
   if (Marking::IsBlack(mark_bit)) return;
   Marking::MarkBlack(mark_bit);
   MemoryChunk::IncrementLiveBytesFromGC(heap_object->address(), size);
-  DCHECK(Marking::IsBlack(mark_bit));
 }
 
 
@@ -251,7 +250,7 @@ class IncrementalMarkingMarkingVisitor
     HeapObject* heap_object = HeapObject::cast(obj);
     MarkBit mark_bit = Marking::MarkBitFrom(heap_object);
     if (Marking::IsWhite(mark_bit)) {
-      mark_bit.Set();
+      Marking::MarkBlack(mark_bit);
       MemoryChunk::IncrementLiveBytesFromGC(heap_object->address(),
                                             heap_object->Size());
       return true;