[heap] Add the free remainder of a black page to the free list.

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 9c5a3b59863ce59db0440fcca8807b76c287b1ea..5f5a1deae781467544cc5d2db744ba433028a4fb 100644
--- a/src/heap/incremental-marking.h
+++ b/src/heap/incremental-marking.h
@@ -215,6 +215,8 @@ class IncrementalMarking {
 
   bool black_allocation() { return black_allocation_; }
 
+  void StartBlackAllocationForTesting() { StartBlackAllocation(); }
+
  private:
   class Observer : public AllocationObserver {
    public: