Don't UnshiftGrey any black objects

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index 322965decdd4e6d57a403f152c85ab8a07c26c16..e8f5c63fe3925a9dcbe3197cf5f31af52b332845 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -244,6 +244,7 @@ class MarkingDeque {
 
   INLINE(void UnshiftGrey(HeapObject* object)) {
     DCHECK(object->IsHeapObject());
+    DCHECK(Marking::IsGrey(Marking::MarkBitFrom(object)));
     if (IsFull()) {
       SetOverflowed();
     } else {
@@ -252,6 +253,19 @@ class MarkingDeque {
     }
   }
 
+  INLINE(void UnshiftBlack(HeapObject* object)) {
+    DCHECK(object->IsHeapObject());
+    DCHECK(Marking::IsBlack(Marking::MarkBitFrom(object)));
+    if (IsFull()) {
+      Marking::BlackToGrey(object);
+      MemoryChunk::IncrementLiveBytesFromGC(object->address(), -object->Size());
+      SetOverflowed();
+    } else {
+      bottom_ = ((bottom_ - 1) & mask_);
+      array_[bottom_] = object;
+    }
+  }
+
   HeapObject** array() { return array_; }
   int bottom() { return bottom_; }
   int top() { return top_; }