Make sweeper threads respect the max_available_threads constraint.

src/heap/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 
 #include "src/allocation.h"
@@ skipping 1298 matching lines @@
 
   IncrementalMarking* incremental_marking() { return &incremental_marking_; }
 
   ExternalStringTable* external_string_table() {
     return &external_string_table_;
   }
 
   // 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);
   void CallGCEpilogueCallbacks(GCType gc_type, GCCallbackFlags flags);
 
   inline bool OldGenerationAllocationLimitReached();
 
   inline void DoScavengeObject(Map* map, HeapObject** slot, HeapObject* obj) {
     scavenging_visitors_table_.GetVisitor(map)(map, slot, obj);
   }
@@ skipping 795 matching lines @@
   VisitorDispatchTable<ScavengingCallback> scavenging_visitors_table_;
 
   MemoryChunk* chunks_queued_for_free_;
 
   base::Mutex relocation_mutex_;
 
   int gc_callbacks_depth_;
 
   bool deserialization_complete_;
 
+  bool concurrent_sweeping_enabled_;
+
   friend class AlwaysAllocateScope;
   friend class Deserializer;
   friend class Factory;
   friend class GCCallbacksScope;
   friend class GCTracer;
   friend class HeapIterator;
   friend class Isolate;
   friend class MarkCompactCollector;
   friend class MarkCompactMarkingVisitor;
   friend class MapCompact;
@@ skipping 448 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 }
 }  // namespace v8::internal
 
 #endif  // V8_HEAP_HEAP_H_