[debugger] allow debug-evaluate to change stack and context values.

test/mjsunit/regress/regress-crbug-323936.js

 // Copyright 2014 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-eval-readonly-locals
+// Flags: --expose-debug-as debug
 
 Debug = debug.Debug;
 
 var step = 0;
 var exception = null;
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     if (step == 0) {
@@ skipping 27 matching lines @@
   assertEquals("argument", e);  // debugger did not overwrite
   function write_2(v) { e = v }
   debugger;
   assertEquals("bar", e);
   assertEquals("modified", x);
 }
 
 f("argument")
 assertNull(exception);
 assertEquals(2, step);