Fix leak in debug mirror cache.

test/mjsunit/regress/regress-debug-context-load.js

Index: test/mjsunit/regress/regress-debug-context-load.js
diff --git a/test/mjsunit/readonly-accessor.js b/test/mjsunit/regress/regress-debug-context-load.js
similarity index 67%
copy from test/mjsunit/readonly-accessor.js
copy to test/mjsunit/regress/regress-debug-context-load.js
index 5a73525fea60d67074d81e392d08d8448f8ae39d..0b3c275f99b016825cd563a700772bbddb7e4bad 100644
--- a/test/mjsunit/readonly-accessor.js
+++ b/test/mjsunit/regress/regress-debug-context-load.js
@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-var foo = {};
-foo.__proto__ = new String("bar");
-foo.length = 20;
+// Flags: --expose-debug-as debug
+
+Debug = debug.Debug;
+Debug.setListener(null);