[debugger] keep scope and context chain for hidden scopes in sync.

test/mjsunit/es6/regress/regress-468661.js

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --expose-debug-as debug
 
 Debug = debug.Debug
 var exception = null;
 var break_count = 0;
 
 var expected_values =
-  [ReferenceError, ReferenceError, 0, 0, 0, 0, 1,
+  [ReferenceError, undefined, 0, 0, 0, 0, 1,
    ReferenceError, ReferenceError];
 
 function listener(event, exec_state, event_data, data) {
   try {
     if (event == Debug.DebugEvent.Break) {
       assertTrue(exec_state.frameCount() != 0, "FAIL: Empty stack trace");
       // Count number of expected breakpoints in this source file.
       if (!break_count) {
         var source_text = exec_state.frame(0).func().script().source();
         expected_breaks = source_text.match(/\/\/\s*Break\s+\d+\./g).length;
@@ skipping 44 matching lines @@
        i++) {   // Break 5.
     let key = i; // Break 3.
     sum += key;   // Break 4.
   }
 }()); // Break 7.
 
 assertNull(exception); // Break 8.
 assertEquals(expected_breaks, break_count);
 
 Debug.setListener(null);