Wait for sweeper threads only if we have to.

src/mark-compact.h

Index: src/mark-compact.h
diff --git a/src/mark-compact.h b/src/mark-compact.h
index 54f28dc9d771c1d1a5d98360b76c694300c4e8d0..6b7c09bab9c2dc4dc91f6ff4d360fb7a99830d9c 100644
--- a/src/mark-compact.h
+++ b/src/mark-compact.h
@@ -670,7 +670,10 @@ class MarkCompactCollector {
 
   bool AreSweeperThreadsActivated();
 
-  bool IsConcurrentSweepingInProgress();
+  // If a paged space is passed in, this method checks if the given space is
+  // swept concurrently. Otherwise, this method checks if concurrent sweeping
+  // is in progress right now on any space.
+  bool IsConcurrentSweepingInProgress(PagedSpace* space = NULL);
 
   void set_sequential_sweeping(bool sequential_sweeping) {
     sequential_sweeping_ = sequential_sweeping;