Fix isolate getter in DebugAgent

src/debug.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 1023 matching lines @@
 class MessageDispatchHelperThread: public Thread {
  public:
   explicit MessageDispatchHelperThread(Isolate* isolate);
   ~MessageDispatchHelperThread();
 
   void Schedule();
 
  private:
   void Run();
 
+  Isolate* isolate_;
   Semaphore* const sem_;
   Mutex* const mutex_;
   bool already_signalled_;
 
   DISALLOW_COPY_AND_ASSIGN(MessageDispatchHelperThread);
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // ENABLE_DEBUGGER_SUPPORT
 
 #endif  // V8_DEBUG_H_