Remove unused field.

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 75 matching lines @@
   }
 
  private:
 #ifdef DEBUG
   int thread_id_;
 #endif
 
   Isolate* isolate_;
   Semaphore* stop_semaphore_;
   Semaphore* input_queue_semaphore_;
-  Semaphore* output_queue_semaphore_;
   UnboundQueue<OptimizingCompiler*> input_queue_;
   UnboundQueue<OptimizingCompiler*> output_queue_;
   volatile AtomicWord stop_thread_;
   volatile Atomic32 queue_length_;
   int64_t time_spent_compiling_;
   int64_t time_spent_total_;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OPTIMIZING_COMPILER_THREAD_H_