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

src/heap/spaces.h

Index: src/heap/spaces.h
diff --git a/src/heap/spaces.h b/src/heap/spaces.h
index fce96772139fde9d79623e20f7c0a2a360e20f07..cd5e9f2c7be4d8b44a6c6e921fd6f76e0aaee442 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -104,19 +104,15 @@ class MarkBit {
 
   inline MarkBit(CellType* cell, CellType mask) : cell_(cell), mask_(mask) {}
 
-  inline CellType* cell() { return cell_; }
-  inline CellType mask() { return mask_; }
-
 #ifdef DEBUG
   bool operator==(const MarkBit& other) {
     return cell_ == other.cell_ && mask_ == other.mask_;
   }
 #endif
 
-  inline void Set() { *cell_ |= mask_; }
-  inline bool Get() { return (*cell_ & mask_) != 0; }
-  inline void Clear() { *cell_ &= ~mask_; }
-
+ private:
+  inline CellType* cell() { return cell_; }
+  inline CellType mask() { return mask_; }
 
   inline MarkBit Next() {
     CellType new_mask = mask_ << 1;
@@ -127,9 +123,14 @@ class MarkBit {
     }
   }
 
- private:
+  inline void Set() { *cell_ |= mask_; }
+  inline bool Get() { return (*cell_ & mask_) != 0; }
+  inline void Clear() { *cell_ &= ~mask_; }
+
   CellType* cell_;
   CellType mask_;
+
+  friend class Marking;
 };