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

src/optimizing-compiler-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 352 matching lines @@
       PrintF(", AST id %d]\n", info->osr_ast_id().ToInt());
     }
     DisposeRecompileJob(stale, false);
   }
   osr_buffer_[osr_buffer_cursor_] = job;
   osr_buffer_cursor_ = (osr_buffer_cursor_ + 1) % osr_buffer_capacity_;
 }
 
 
 #ifdef DEBUG
+bool OptimizingCompilerThread::IsOptimizerThread(Isolate* isolate) {
+  return isolate->concurrent_recompilation_enabled() &&
+         isolate->optimizing_compiler_thread()->IsOptimizerThread();
+}
+
+
 bool OptimizingCompilerThread::IsOptimizerThread() {
-  if (!FLAG_concurrent_recompilation) return false;
   LockGuard<Mutex> lock_guard(&thread_id_mutex_);
   return ThreadId::Current().ToInteger() == thread_id_;
 }
 #endif
 
 
 } }  // namespace v8::internal