[heap] Use shared markbits in the concurrent marker.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 6d7a5da1d31e69ce6a2e1b3a42cb7fc81c283b7d..2d6f185d71f2834d2900af1bd6c90520676e3eb5 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1525,6 +1525,9 @@ void HeapObject::set_map_no_write_barrier(Map* value) {
   set_map_word(MapWord::FromMap(value));
 }
 
+HeapObject** HeapObject::map_slot() {
+  return reinterpret_cast<HeapObject**>(FIELD_ADDR(this, kMapOffset));
+}
 
 MapWord HeapObject::map_word() const {
   return MapWord(