[debugger] remove step count parameter from prepare step.

test/mjsunit/es6/debug-step-into-class-extends.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
 
 'use strict';
 
 var Debug = debug.Debug
 
 var done = false;
 var stepCount = 0;
 
 function listener(event, execState, eventData, data) {
   if (event == Debug.DebugEvent.Break) {
     if (!done) {
-      execState.prepareStep(Debug.StepAction.StepInto);
+      execState.prepareStep(Debug.StepAction.StepIn);
       var s = execState.frame().sourceLineText();
       assertTrue(s.indexOf('// ' + stepCount + '.') !== -1);
       stepCount++;
     }
   }
 };
 
 Debug.setListener(listener);
 
 function GetBase() {
   var x = 1;   // 1.
   var y = 2;   // 2.
   done = true; // 3.
   return null;
 }
 
 function f() {
   class Derived extends GetBase() {} // 0.
 }
 
 var bp = Debug.setBreakPoint(f, 0);
 f();
 assertEquals(4, stepCount);
 
 Debug.setListener(null);