Access old space marking bits from runtime only when incremental marking is in MARKING state.

src/mark-compact.h

Index: src/mark-compact.h
diff --git a/src/mark-compact.h b/src/mark-compact.h
index 40b73fb7dd9f0584fea0661fa3608863e6a55f9e..0ebe8a0f747846193bfd0002cccd8bec247df4a0 100644
--- a/src/mark-compact.h
+++ b/src/mark-compact.h
@@ -110,8 +110,7 @@ class Marking {
     markbit.Next().Set();
   }
 
-  // Returns true if the the object whose mark is transferred is marked black.
-  bool TransferMark(Address old_start, Address new_start);
+  void TransferMark(Address old_start, Address new_start);
 
 #ifdef DEBUG
   enum ObjectColor {