[debugger] Further stepping support in test wrapper

test/debugger/debug/debug-evaluate-modify-this.js

 // Copyright 2016 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 f = () => { debugger; }
 var g = function() { debugger; }
 var h = (function() { return () => { debugger; }; }).call({});
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     assertThrows(() => exec_state.frame(0).evaluate("this = 2"));
   } catch (e) {
     exception = e;
     print("Caught something. " + e + " " + e.stack);
   };
 };
 
 Debug.setListener(listener);
 
 f();
 g();
 g.call({});
 h();
 
 Debug.setListener(null);
 assertNull(exception);