Allow main thread to contribute to the sweeping phase.

src/sweeper-thread.cc

Index: src/sweeper-thread.cc
diff --git a/src/sweeper-thread.cc b/src/sweeper-thread.cc
index 9ce5ec4d85736657f826b0c42b0f40fad181d6be..83b4ec1c9ddca0a9630ee768ac51030c337b5d3c 100644
--- a/src/sweeper-thread.cc
+++ b/src/sweeper-thread.cc
@@ -41,8 +41,10 @@ void SweeperThread::Run() {
       return;
     }
 
-    collector_->SweepInParallel(heap_->old_data_space());
-    collector_->SweepInParallel(heap_->old_pointer_space());
+    collector_->SweepInParallel<MarkCompactCollector::SWEEP_IN_PARALLEL>(
+        heap_->old_data_space(), 0);
+    collector_->SweepInParallel<MarkCompactCollector::SWEEP_IN_PARALLEL>(
+        heap_->old_pointer_space(), 0);
     end_sweeping_semaphore_.Signal();
   }
 }