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

src/sweeper-thread.cc

Index: src/sweeper-thread.cc
diff --git a/src/sweeper-thread.cc b/src/sweeper-thread.cc
index 58c684a54f62c40f0f1aa51f761b99c0f64189dc..6f3baed11f8f111d91e1caf6c61685b043489b7f 100644
--- a/src/sweeper-thread.cc
+++ b/src/sweeper-thread.cc
@@ -105,4 +105,14 @@ void SweeperThread::StartSweeping() {
 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