Wait for sweeper threads only if we have to.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 812fec127ad34973d6d2a0115fc51e98a9cbab72..5c96c50f560d411eda28c2d68f1610ea859fbdb8 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -1297,7 +1297,7 @@ static void VerifyNonPointerSpacePointers(Heap* heap) {
 
   // The old data space was normally swept conservatively so that the iterator
   // doesn't work, so we normally skip the next bit.
-  if (!heap->old_data_space()->was_swept_conservatively()) {
+  if (heap->old_data_space()->is_iterable()) {
     HeapObjectIterator data_it(heap->old_data_space());
     for (HeapObject* object = data_it.Next();
          object != NULL; object = data_it.Next())
@@ -4246,8 +4246,8 @@ STRUCT_LIST(MAKE_CASE)
 
 
 bool Heap::IsHeapIterable() {
-  return (!old_pointer_space()->was_swept_conservatively() &&
-          !old_data_space()->was_swept_conservatively() &&
+  return (old_pointer_space()->is_iterable() &&
+          old_data_space()->is_iterable() &&
           new_space_top_after_last_gc_ == new_space()->top());
 }