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

src/sweeper-thread.cc

 // 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 87 matching lines @@
 
 
 void SweeperThread::StartSweeping() {
   start_sweeping_semaphore_.Signal();
 }
 
 
 void SweeperThread::WaitForSweeperThread() {
   end_sweeping_semaphore_.Wait();
 }
+
+
+int SweeperThread::NumberOfThreads(int max_available) {
+  if (!FLAG_concurrent_sweeping && !FLAG_parallel_sweeping) return 0;
+  if (FLAG_sweeper_threads > 0) return FLAG_sweeper_threads;
+  if (FLAG_concurrent_sweeping) return max_available - 1;
+  ASSERT(FLAG_parallel_sweeping);
+  return max_available;
+}
+
 } }  // namespace v8::internal