Move sweeping pending status field of concurrent sweepers to MarkCompactCollector.

src/mark-compact.h

Index: src/mark-compact.h
diff --git a/src/mark-compact.h b/src/mark-compact.h
index 2613a3e13516ec726a207280b3970fd0b5f1d9fb..7a328576508f092c228e72f77534cd4b7ffe03ff 100644
--- a/src/mark-compact.h
+++ b/src/mark-compact.h
@@ -740,6 +740,9 @@ class MarkCompactCollector {
 
   bool was_marked_incrementally_;
 
+  // True if concurrent or parallel sweeping is currently in progress.
+  bool sweeping_pending_;
+
   // A pointer to the current stack-allocated GC tracer object during a full
   // collection (NULL before and after).
   GCTracer* tracer_;