[debugger] remove step count parameter from prepare step.

test/mjsunit/debug-stepin-construct-call.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
 
 var break_count = 0;
 var exception = null;
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     var source_line = exec_state.frame(0).sourceLineText();
     print(source_line);
-    exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+    exec_state.prepareStep(Debug.StepAction.StepIn);
     break_count++;
   } catch (e) {
     exception = e;
   }
 }
 
 var Debug = debug.Debug;
 Debug.setListener(listener);
 
 
 function f() {
   this.x = 1;
 }
 
 function g() {
   new f();
 }
 
 Debug.setBreakPoint(g, 6, Debug.BreakPositionAlignment.BreakPosition);
 print(Debug.showBreakPoints(g, undefined,
       Debug.BreakPositionAlignment.BreakPosition));
 
 g();
 Debug.setListener(null);
 
 assertEquals(6, break_count);
 assertNull(exception);