Make number of available threads isolate-dependent and expose it to ResourceConstraints.

src/sweeper-thread.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 33 matching lines @@
  public:
   explicit SweeperThread(Isolate* isolate);
   ~SweeperThread() {}
 
   void Run();
   void Stop();
   void StartSweeping();
   void WaitForSweeperThread();
   intptr_t StealMemory(PagedSpace* space);
 
+  static int NumberOfThreads(int max_available);
+
  private:
   Isolate* isolate_;
   Heap* heap_;
   MarkCompactCollector* collector_;
   Semaphore start_sweeping_semaphore_;
   Semaphore end_sweeping_semaphore_;
   Semaphore stop_semaphore_;
   FreeList free_list_old_data_space_;
   FreeList free_list_old_pointer_space_;
   FreeList private_free_list_old_data_space_;
   FreeList private_free_list_old_pointer_space_;
   volatile AtomicWord stop_thread_;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_SWEEPER_THREAD_H_