Wait for sweeper threads only if we have to.

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index 8846f19bb0dbaeec80192bf9709b3e3f1029cf2c..194663a92c24a2d7322754a5e2de8480f4ca8651 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -647,8 +647,9 @@ bool MarkCompactCollector::AreSweeperThreadsActivated() {
 }
 
 
-bool MarkCompactCollector::IsConcurrentSweepingInProgress() {
-  return sweeping_pending_;
+bool MarkCompactCollector::IsConcurrentSweepingInProgress(PagedSpace* space) {
+  return (space == NULL || space->was_swept_concurrently()) &&
+      sweeping_pending_;
 }
 
 
@@ -4081,6 +4082,7 @@ void MarkCompactCollector::SweepSpace(PagedSpace* space, SweeperType sweeper) {
   space->set_was_swept_conservatively(sweeper == CONSERVATIVE ||
                                       sweeper == PARALLEL_CONSERVATIVE ||
                                       sweeper == CONCURRENT_CONSERVATIVE);
+  space->set_was_swept_concurrently(sweeper == CONCURRENT_CONSERVATIVE);
   space->ClearStats();
 
   // We defensively initialize end_of_unswept_pages_ here with the first page