[heap] Reland "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 a3f87c4b24eb773fef3367ca5ec86c94353edb3f..9e91da2cdc41a5a4a9c9019fc0c947e5db64347b 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(