Fix PagedSpace size accounting.

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index 08cc80323789d35493271137ba2ad59c32707b72..9df3e6f1446280a9ad3564e4db24de9bce7d5b22 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -3947,6 +3947,7 @@ static intptr_t Free(PagedSpace* space,
   if (mode == MarkCompactCollector::SWEEP_ON_MAIN_THREAD) {
     return space->Free(start, size);
   } else {
+    // TODO(hpayer): account for wasted bytes in concurrent sweeping too.
     return size - free_list->Free(start, size);
   }
 }