Use the incremental root marking visitor for overapproximating the weak closure

src/heap/incremental-marking.h

Index: src/heap/incremental-marking.h
diff --git a/src/heap/incremental-marking.h b/src/heap/incremental-marking.h
index d6dfe17c7f2472ebb9ce76bdab694e1cb5a00708..0b16f3713fcd9373f5c843c4fd95a71524a28a8d 100644
--- a/src/heap/incremental-marking.h
+++ b/src/heap/incremental-marking.h
@@ -189,6 +189,10 @@ class IncrementalMarking {
 
   bool IsIdleMarkingDelayCounterLimitReached();
 
+  // Creates a visitor for marking roots using this incremental marking
+  // instance. Caller takes ownership.
+  ObjectVisitor* CreateRootMarkingVisitor();
+
  private:
   int64_t SpaceLeftInOldSpace();