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

src/heap/mark-compact-inl.h

Index: src/heap/mark-compact-inl.h
diff --git a/src/heap/mark-compact-inl.h b/src/heap/mark-compact-inl.h
index 66b0a59cec6c0b32ba3cf2147e0af2e55bd81f98..c38d20db2a9080142d76b7c00f0804ac0b577a84 100644
--- a/src/heap/mark-compact-inl.h
+++ b/src/heap/mark-compact-inl.h
@@ -29,10 +29,9 @@ void MarkCompactCollector::SetFlags(int flags) {
 
 void MarkCompactCollector::MarkObject(HeapObject* obj, MarkBit mark_bit) {
   DCHECK(Marking::MarkBitFrom(obj) == mark_bit);
-  if (!mark_bit.Get()) {
-    mark_bit.Set();
+  if (Marking::IsWhite(mark_bit)) {
+    Marking::WhiteToBlack(mark_bit);
     MemoryChunk::IncrementLiveBytesFromGC(obj->address(), obj->Size());
-    DCHECK(IsMarked(obj));
     DCHECK(obj->GetIsolate()->heap()->Contains(obj));
     marking_deque_.PushBlack(obj);
   }
@@ -40,9 +39,9 @@ void MarkCompactCollector::MarkObject(HeapObject* obj, MarkBit mark_bit) {
 
 
 void MarkCompactCollector::SetMark(HeapObject* obj, MarkBit mark_bit) {
-  DCHECK(!mark_bit.Get());
+  DCHECK(Marking::IsWhite(mark_bit));
   DCHECK(Marking::MarkBitFrom(obj) == mark_bit);
-  mark_bit.Set();
+  Marking::WhiteToBlack(mark_bit);
   MemoryChunk::IncrementLiveBytesFromGC(obj->address(), obj->Size());
 }
 
@@ -50,7 +49,7 @@ void MarkCompactCollector::SetMark(HeapObject* obj, MarkBit mark_bit) {
 bool MarkCompactCollector::IsMarked(Object* obj) {
   DCHECK(obj->IsHeapObject());
   HeapObject* heap_object = HeapObject::cast(obj);
-  return Marking::MarkBitFrom(heap_object).Get();
+  return Marking::IsMarked(Marking::MarkBitFrom(heap_object));
 }