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

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index c6800993c00fabd5ac963a62e51b4d80a90ec28b..2bd53df773d49e3a3f74f96480c0c1eb9a6114e7 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -50,7 +50,10 @@ class Marking {
 
   // White markbits: 00 - this is required by the mark bit clearer.
   static const char* kWhiteBitPattern;
-  INLINE(static bool IsWhite(MarkBit mark_bit)) { return !mark_bit.Get(); }
+  INLINE(static bool IsWhite(MarkBit mark_bit)) {
+    DCHECK(!IsImpossible(mark_bit));
+    return !mark_bit.Get();
+  }
 
   // Grey markbits: 11
   static const char* kGreyBitPattern;
@@ -58,19 +61,51 @@ class Marking {
     return mark_bit.Get() && mark_bit.Next().Get();
   }
 
+  // IsBlackOrGrey assumes that the first bit is set for black or grey
+  // objects.
+  INLINE(static bool IsBlackOrGrey(MarkBit mark_bit)) { return mark_bit.Get(); }
+
   INLINE(static void MarkBlack(MarkBit mark_bit)) {
     mark_bit.Set();
     mark_bit.Next().Clear();
   }
 
-  INLINE(static void BlackToGrey(MarkBit markbit)) { markbit.Next().Set(); }
+  INLINE(static void MarkWhite(MarkBit mark_bit)) {
+    mark_bit.Clear();
+    mark_bit.Next().Clear();
+  }
+
+  INLINE(static void BlackToWhite(MarkBit markbit)) {
+    DCHECK(IsBlack(markbit));
+    markbit.Clear();
+  }
+
+  INLINE(static void GreyToWhite(MarkBit markbit)) {
+    DCHECK(IsGrey(markbit));
+    markbit.Clear();
+    markbit.Next().Clear();
+  }
+
+  INLINE(static void BlackToGrey(MarkBit markbit)) {
+    DCHECK(IsBlack(markbit));
+    markbit.Next().Set();
+  }
 
   INLINE(static void WhiteToGrey(MarkBit markbit)) {
+    DCHECK(IsWhite(markbit));
     markbit.Set();
     markbit.Next().Set();
   }
 
-  INLINE(static void GreyToBlack(MarkBit markbit)) { markbit.Next().Clear(); }
+  INLINE(static void WhiteToBlack(MarkBit markbit)) {
+    DCHECK(IsWhite(markbit));
+    markbit.Set();
+  }
+
+  INLINE(static void GreyToBlack(MarkBit markbit)) {
+    DCHECK(IsGrey(markbit));
+    markbit.Next().Clear();
+  }
 
   INLINE(static void BlackToGrey(HeapObject* obj)) {
     BlackToGrey(MarkBitFrom(obj));
@@ -81,6 +116,10 @@ class Marking {
     markbit.Next().Set();
   }
 
+  static void SetAllMarkBitsInRange(MarkBit start, MarkBit end);
+  static void ClearAllMarkBitsOfCellsContainedInRange(MarkBit start,
+                                                      MarkBit end);
+
   void TransferMark(Address old_start, Address new_start);
 
 #ifdef DEBUG