Separate markbits from heap object map words into bitmaps.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 154a4f1c1f286b793697f1b6678debfb38ad8a42..eaf6e1191ea6cdfc7cf8dc19e34134ff7c0eba35 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -867,6 +867,7 @@ class Heap : public AllStatic {
 
   // Returns whether the object resides in new space.
   static inline bool InNewSpace(Object* object);
+  static inline bool InNewSpace(Address addr);
   static inline bool InFromSpace(Object* object);
   static inline bool InToSpace(Object* object);
 
@@ -1942,10 +1943,6 @@ class GCTracer BASE_EMBEDDED {
   // cleared.  Will be zero on a scavenge collection.
   int marked_count_;
 
-  // The count from the end of the previous full GC.  Will be zero if there
-  // was no previous full GC.
-  int previous_marked_count_;
-
   // Amounts of time spent in different scopes during GC.
   double scopes_[Scope::kNumberOfScopes];