When adjusting page's live bytes counter from the mutator adjust owners unswept free bytes counter.

src/spaces-inl.h

Index: src/spaces-inl.h
diff --git a/src/spaces-inl.h b/src/spaces-inl.h
index bd544ce639e13cadc4405a65727c37cbfe139d4e..d0cddebf787de98cc1171b57eeaf8d125a28f102 100644
--- a/src/spaces-inl.h
+++ b/src/spaces-inl.h
@@ -332,7 +332,7 @@ void NewSpace::ShrinkStringAtAllocationBoundary(String* string, int length) {
   string->set_length(length);
   if (Marking::IsBlack(Marking::MarkBitFrom(string))) {
     int delta = static_cast<int>(old_top - allocation_info_.top);
-    MemoryChunk::IncrementLiveBytes(string->address(), -delta);
+    MemoryChunk::IncrementLiveBytesFromMutator(string->address(), -delta);
   }
 }