Wait for sweeper threads in EnsureSweeperProgress() only if the main thread finished its sweeping p…

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index ea797a5866a63fb26dd46b76baeadb85effeccc6..f85fa694b25af717642c0b1908ff96c33ab2ad71 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -67,6 +67,7 @@ MarkCompactCollector::MarkCompactCollector() :  // NOLINT
       compacting_(false),
       was_marked_incrementally_(false),
       sweeping_pending_(false),
+      sequential_sweeping_(false),
       tracer_(NULL),
       migration_slots_buffer_(NULL),
       heap_(NULL),
@@ -3895,7 +3896,7 @@ void MarkCompactCollector::SweepSpaces() {
   // the map space last because freeing non-live maps overwrites them and
   // the other spaces rely on possibly non-live maps to get the sizes for
   // non-live objects.
-
+  SequentialSweepingScope scope(this);
   SweepSpace(heap()->old_pointer_space(), how_to_sweep);
   SweepSpace(heap()->old_data_space(), how_to_sweep);