Wait for sweeper threads only if we have to.

src/spaces.h

Index: src/spaces.h
diff --git a/src/spaces.h b/src/spaces.h
index a940eed5a6c4ae463fe10e173ce7a8321dd230bf..71b70546833824ea562cfdecf6b2f0336a5166b4 100644
--- a/src/spaces.h
+++ b/src/spaces.h
@@ -1902,8 +1902,11 @@ class PagedSpace : public Space {
   static void ResetCodeStatistics(Isolate* isolate);
 #endif
 
-  bool was_swept_conservatively() { return was_swept_conservatively_; }
-  void set_was_swept_conservatively(bool b) { was_swept_conservatively_ = b; }
+  bool is_iterable() { return is_iterable_; }
+  void set_is_iterable(bool b) { is_iterable_ = b; }
+
+  bool is_swept_concurrently() { return is_swept_concurrently_; }
+  void set_is_swept_concurrently(bool b) { is_swept_concurrently_ = b; }
 
   // Evacuation candidates are swept by evacuator.  Needs to return a valid
   // result before _and_ after evacuation has finished.
@@ -1986,7 +1989,11 @@ class PagedSpace : public Space {
   // Normal allocation information.
   AllocationInfo allocation_info_;
 
-  bool was_swept_conservatively_;
+  // This space was swept precisely, hence it is iterable.
+  bool is_iterable_;
+
+  // This space is currently swept by sweeper threads.
+  bool is_swept_concurrently_;
 
   // The number of free bytes which could be reclaimed by advancing the
   // concurrent sweeper threads.  This is only an estimation because concurrent