Wait for sweeper threads only if we have to.

src/heap-snapshot-generator.cc

Index: src/heap-snapshot-generator.cc
diff --git a/src/heap-snapshot-generator.cc b/src/heap-snapshot-generator.cc
index 80d3c1061450bb3026e8b2b4eeffd77335745ddf..df1e34790ce182001bdcb430d75c13558f7d8086 100644
--- a/src/heap-snapshot-generator.cc
+++ b/src/heap-snapshot-generator.cc
@@ -2586,13 +2586,12 @@ bool HeapSnapshotGenerator::GenerateSnapshot() {
 
 #ifdef VERIFY_HEAP
   Heap* debug_heap = heap_;
-  CHECK(!debug_heap->old_data_space()->was_swept_conservatively());
-  CHECK(!debug_heap->old_pointer_space()->was_swept_conservatively());
-  CHECK(!debug_heap->code_space()->was_swept_conservatively());
-  CHECK(!debug_heap->cell_space()->was_swept_conservatively());
-  CHECK(!debug_heap->property_cell_space()->
-        was_swept_conservatively());
-  CHECK(!debug_heap->map_space()->was_swept_conservatively());
+  CHECK(debug_heap->old_data_space()->is_iterable());
+  CHECK(debug_heap->old_pointer_space()->is_iterable());
+  CHECK(debug_heap->code_space()->is_iterable());
+  CHECK(debug_heap->cell_space()->is_iterable());
+  CHECK(debug_heap->property_cell_space()->is_iterable());
+  CHECK(debug_heap->map_space()->is_iterable());
 #endif
 
 #ifdef VERIFY_HEAP