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..1de8d255580478adc04496d3f71f3d28a0e6c0a2 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -208,7 +208,7 @@ static void VerifyEvacuation(PagedSpace* space) {
   // TODO(hpayer): Bring back VerifyEvacuation for parallel-concurrently
   // swept pages.
   if ((FLAG_concurrent_sweeping || FLAG_parallel_sweeping) &&
-      space->was_swept_conservatively()) return;
+      !space->is_iterable()) return;
   PageIterator it(space);
 
   while (it.has_next()) {
@@ -647,8 +647,9 @@ bool MarkCompactCollector::AreSweeperThreadsActivated() {
 }
 
 
-bool MarkCompactCollector::IsConcurrentSweepingInProgress() {
-  return sweeping_pending_;
+bool MarkCompactCollector::IsConcurrentSweepingInProgress(PagedSpace* space) {
+  return (space == NULL || space->is_swept_concurrently()) &&
+      sweeping_pending_;
 }
 
 
@@ -2044,7 +2045,7 @@ int MarkCompactCollector::DiscoverAndEvacuateBlackObjectsOnPage(
 static void DiscoverGreyObjectsInSpace(Heap* heap,
                                        MarkingDeque* marking_deque,
                                        PagedSpace* space) {
-  if (!space->was_swept_conservatively()) {
+  if (space->is_iterable()) {
     HeapObjectIterator it(space);
     DiscoverGreyObjectsWithIterator(heap, marking_deque, &it);
   } else {
@@ -4078,9 +4079,8 @@ void MarkCompactCollector::SweepInParallel(PagedSpace* space) {
 
 
 void MarkCompactCollector::SweepSpace(PagedSpace* space, SweeperType sweeper) {
-  space->set_was_swept_conservatively(sweeper == CONSERVATIVE ||
-                                      sweeper == PARALLEL_CONSERVATIVE ||
-                                      sweeper == CONCURRENT_CONSERVATIVE);
+  space->set_is_iterable(sweeper == PRECISE);
+  space->set_is_swept_concurrently(sweeper == CONCURRENT_CONSERVATIVE);
   space->ClearStats();
 
   // We defensively initialize end_of_unswept_pages_ here with the first page