Fix leak in debug mirror cache.

test/mjsunit/debug-scripts-request.js

 // 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 90 matching lines @@
 // Add the debug event listener.
 Debug.setListener(listener);
 
 // Call debugger to invoke the debug event listener.
 debugger;
 
 // Make sure that the debug event listener vas invoked with no exceptions.
 assertTrue(listenerComplete,
            "listener did not run to completion, exception: " + exception);
 assertFalse(exception, "exception in listener")
+
+Debug.setListener(null);