Debugger: require debugger to be active when dealing with breaks.

test/mjsunit/regress/regress-crbug-424142.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
 
 (function outer() {
   var C = (function C_() {
     var y = 1;
     function CC() {
       this.x = 0;
     }
     CC.prototype.f = function CCf() {
       this.x += y;
       return this.x;
     };
     return CC;
   })();
 
   var c = new C(0);
 })
 
 function sentinel() {}
 
 Debug = debug.Debug;
+Debug.setListener(function(){});
 
 var script = Debug.findScript(sentinel);
 var line = 14;
 var line_start = Debug.findScriptSourcePosition(script, line, 0);
 var line_end = Debug.findScriptSourcePosition(script, line + 1, 0) - 1;
 var actual = Debug.setBreakPointByScriptIdAndPosition(
                  script.id, line_start).actual_position;
 // Make sure the actual break position is within the line where we set
 // the break point.
 assertTrue(line_start <= actual);
 assertTrue(actual <= line_end);
+
+Debug.setListener(null);