Move the temporary private freelist closer to where it's used

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index c7e98b7bab704c79eb6889437fe260644fdc9604..a64b3bdda1ae7df4b79b7aa49121bce8f504843c 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -3971,15 +3971,15 @@ intptr_t MarkCompactCollector::SweepConservatively(PagedSpace* space,
 
 
 void MarkCompactCollector::SweepInParallel(PagedSpace* space,
-                                           FreeList* private_free_list,
                                            FreeList* free_list) {
   PageIterator it(space);
+  FreeList private_free_list(space);
   while (it.has_next()) {
     Page* p = it.next();
 
     if (p->TryParallelSweeping()) {
-      SweepConservatively<SWEEP_IN_PARALLEL>(space, private_free_list, p);
-      free_list->Concatenate(private_free_list);
+      SweepConservatively<SWEEP_IN_PARALLEL>(space, &private_free_list, p);
+      free_list->Concatenate(&private_free_list);
     }
   }
 }