[heap] Fix computation of marking progress in incremental finalization.

src/heap/sequential-marking-deque.h

Index: src/heap/sequential-marking-deque.h
diff --git a/src/heap/sequential-marking-deque.h b/src/heap/sequential-marking-deque.h
index bd579ba713daad719d206db73d5eef32d81871d3..89b762f3ce435397360d324f764b85fff1c7806b 100644
--- a/src/heap/sequential-marking-deque.h
+++ b/src/heap/sequential-marking-deque.h
@@ -47,6 +47,11 @@ class SequentialMarkingDeque {
 
   inline bool IsEmpty() { return top_ == bottom_; }
 
+  int Size() {
+    // Return (top - bottom + capacity) % capacity, where capacity = mask + 1.
+    return (top_ - bottom_ + mask_ + 1) & mask_;
+  }
+
   bool overflowed() const { return overflowed_; }
 
   void ClearOverflowed() { overflowed_ = false; }
@@ -114,8 +119,6 @@ class SequentialMarkingDeque {
     top_ = new_top;
   }
 
-  int top() { return top_; }
-
  private:
   // This task uncommits the marking_deque backing store if
   // markin_deque->in_use_ is false.