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

src/optimizing-compiler-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 78 matching lines @@
     return input_queue_length_ < input_queue_capacity_;
   }
 
   inline void AgeBufferedOsrJobs() {
     // Advance cursor of the cyclic buffer to next empty slot or stale OSR job.
     // Dispose said OSR job in the latter case.  Calling this on every GC
     // should make sure that we do not hold onto stale jobs indefinitely.
     AddToOsrBuffer(NULL);
   }
 
+  static bool Enabled(int max_available) {
+    return (FLAG_concurrent_recompilation && max_available > 1);
+  }
+
 #ifdef DEBUG
+  static bool IsOptimizerThread(Isolate* isolate);
   bool IsOptimizerThread();
 #endif
 
  private:
   enum StopFlag { CONTINUE, STOP, FLUSH };
 
   void FlushInputQueue(bool restore_function_code);
   void FlushOutputQueue(bool restore_function_code);
   void FlushOsrBuffer(bool restore_function_code);
   void CompileNext();
@@ skipping 40 matching lines @@
 
   int osr_hits_;
   int osr_attempts_;
 
   int blocked_jobs_;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OPTIMIZING_COMPILER_THREAD_H_