[Isolates] Fix compile under Win32 and presumably for FreeBSD, OpenBS...

src/v8threads.h

 // Copyright 2008 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 143 matching lines @@
   // the handling of a preemption request.
   static void PreemptionReceived();
 
  private:
   explicit ContextSwitcher(Isolate* isolate, int every_n_ms);
 
   void Run();
 
   bool keep_going_;
   int sleep_ms_;
-  Isolate* isolate_;
-
-  static ContextSwitcher* singleton_;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_V8THREADS_H_