[debugger] remove step count parameter from prepare step.

test/mjsunit/es6/debug-break-default-constructor.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
 
 "use strict";
 
 var Debug = debug.Debug;
 var exception = null;
 var super_called = false;
 var step_count = 0;
 
 function listener(event, execState, eventData, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
-    execState.prepareStep(Debug.StepAction.StepInto);
+    execState.prepareStep(Debug.StepAction.StepIn);
     var s = execState.frame().sourceLineText();
     step_count++;
     assertTrue(s.indexOf('// ' + step_count + '.') >= 0);
   } catch (e) {
     exception = e;
   }
 }
 
 class Base {
   constructor() {
     var x = 1;     // 2.
   }                // 3.
 }
 
 class Derived extends Base {}  // 1. // 4.
 
 Debug.setListener(listener);
 var bp = Debug.setBreakPoint(Derived, 0);
 
 new Derived();
 
 Debug.setListener(null);  // 5.
 
 assertNull(exception);
 assertEquals(5, step_count);