Simplify implementation of Mutex.

src/v8threads.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 101 matching lines @@
   ThreadState* GetFreeThreadState();
 
  private:
   ThreadManager();
   ~ThreadManager();
 
   void DeleteThreadStateList(ThreadState* anchor);
 
   void EagerlyArchiveThread();
 
-  Mutex* mutex_;
+  Mutex mutex_;
   ThreadId mutex_owner_;
   ThreadId lazily_archived_thread_;
   ThreadState* lazily_archived_thread_state_;
 
   // In the following two lists there is always at least one object on the list.
   // The first object is a flying anchor that is only there to simplify linking
   // and unlinking.
   // Head of linked list of free states.
   ThreadState* free_anchor_;
   // Head of linked list of states in use.
@@ skipping 30 matching lines @@
   void Run();
 
   bool keep_going_;
   int sleep_ms_;
   Isolate* isolate_;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_V8THREADS_H_