Make sweeper threads respect the max_available_threads constraint.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index a55cb6dfcb000c72090d367ba6be416e551cf355..3720c82fff9aa7d1a617e56a1e8989b18ea3fb6b 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1316,6 +1316,8 @@ class Heap {
   // Returns the current sweep generation.
   int sweep_generation() { return sweep_generation_; }
 
+  bool concurrent_sweeping_enabled() { return concurrent_sweeping_enabled_; }
+
   inline Isolate* isolate();
 
   void CallGCPrologueCallbacks(GCType gc_type, GCCallbackFlags flags);
@@ -2131,6 +2133,8 @@ class Heap {
 
   bool deserialization_complete_;
 
+  bool concurrent_sweeping_enabled_;
+
   friend class AlwaysAllocateScope;
   friend class Deserializer;
   friend class Factory;