[debugger] remove step count parameter from prepare step.

test/mjsunit/regress/regress-crbug-568477-4.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 --allow-natives-syntax
 
 var Debug = debug.Debug;
 var expected =
     ["debugger;", "var x = y;", "var b = 2;", "Debug.setListener(null);"];
 var log = [];
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     log.push(exec_state.frame(0).sourceLineText().trimLeft());
-    exec_state.prepareStep(Debug.StepAction.StepNext, 1);
+    exec_state.prepareStep(Debug.StepAction.StepNext);
   } catch (e) {
     %AbortJS(e + "\n" + e.stack);
   }
 }
 
 Debug.setListener(listener);
 
 function f() {
   var a = 1;
   debugger;
   var x = y;
   print(x);
 }
 
 try {
   %Call(f, {});
 } catch (e) {
   var b = 2;
 }
 
 Debug.setListener(null);
 
 assertEquals(expected, log);