Remove unnecessary mutex.

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 121 matching lines @@
   RecompileJob** osr_buffer_;
   // Cursor for the cyclic buffer.
   int osr_cursor_;
   int osr_buffer_size_;
 
   volatile AtomicWord stop_thread_;
   volatile Atomic32 queue_length_;
   TimeDelta time_spent_compiling_;
   TimeDelta time_spent_total_;
 
-  // TODO(yangguo): remove this once the memory leak has been figured out.
-  Mutex queue_mutex_;
   int osr_hits_;
   int osr_attempts_;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OPTIMIZING_COMPILER_THREAD_H_