[heap] Minor MC: Add marking

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 015da615b2786f7ccfaf5f296a0384fb05fb30df..d16c18d8d90fdfc9a3b58b7e5238d47bdb9bbe07 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -675,6 +675,8 @@ class Heap {
   // they are in new space.
   static bool RootCanBeWrittenAfterInitialization(RootListIndex root_index);
 
+  static bool IsUnmodifiedHeapObject(Object** p);
+
   // Zapping is needed for verify heap, and always done in debug builds.
   static inline bool ShouldZapGarbage() {
 #ifdef DEBUG
@@ -789,6 +791,9 @@ class Heap {
   Object* encountered_weak_collections() const {
     return encountered_weak_collections_;
   }
+  void VisitEncounteredWeakCollections(ObjectVisitor* visitor) {
+    visitor->VisitPointer(&encountered_weak_collections_);
+  }
 
   void set_encountered_weak_cells(Object* weak_cell) {
     encountered_weak_cells_ = weak_cell;