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 0272d59944149009fe3c01a6941052520772a1ec..140325941e093d632b4a211a98bda9ffe32e1d63 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -105,21 +105,18 @@ class MarkBit {
   inline MarkBit(CellType* cell, CellType mask, bool data_only)
       : cell_(cell), mask_(mask), data_only_(data_only) {}
 
-  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_; }
-
   inline bool data_only() { return data_only_; }
 
+ private:
+  inline CellType* cell() { return cell_; }
+  inline CellType mask() { return mask_; }
+
   inline MarkBit Next() {
     CellType new_mask = mask_ << 1;
     if (new_mask == 0) {
@@ -129,7 +126,10 @@ class MarkBit {
     }
   }
 
- private:
+  inline void Set() { *cell_ |= mask_; }
+  inline bool Get() { return (*cell_ & mask_) != 0; }
+  inline void Clear() { *cell_ &= ~mask_; }
+
   CellType* cell_;
   CellType mask_;
   // This boolean indicates that the object is in a data-only space with no
@@ -137,6 +137,8 @@ class MarkBit {
   // It is expected that this field is inlined and turned into control flow
   // at the place where the MarkBit object is created.
   bool data_only_;
+
+  friend class Marking;
 };