Integrate concurrent sweeping with incremental marking.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index f79b0eabb71132bbcda0ce10439e53e41bbc16f7..9177b40f663bd84aa2d1fea1c515ae3c9fad0b36 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1697,6 +1697,12 @@ class Heap {
     return sweeping_complete;
   }
 
+  bool EnsureSweepersProgressed(int step_size) {
+    bool sweeping_complete = old_data_space()->EnsureSweeperProgress(step_size);
+    sweeping_complete &= old_pointer_space()->EnsureSweeperProgress(step_size);
+    return sweeping_complete;
+  }
+
   ExternalStringTable* external_string_table() {
     return &external_string_table_;
   }