[heap] Properly account for wasted bytes.

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index bf2849fb7f7ce275f0aff03d24b78371702724f6..d161b1853d8afb662193e416dfa471380f93164b 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -3553,7 +3553,6 @@ static intptr_t Free(PagedSpace* space, FreeList* free_list, Address start,
     DCHECK(free_list == NULL);
     return space->Free(start, size);
   } else {
-    // TODO(hpayer): account for wasted bytes in concurrent sweeping too.
     return size - free_list->Free(start, size);
   }
 }