[debugger] remove step count parameter from prepare step.

test/mjsunit/regress/regress-crbug-405922.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: --allow-natives-syntax --expose-debug-as debug
 
 Debug = debug.Debug
+var exception = null;
 
 function listener(event, exec_state, event_data, data) {
   try {
     if (event == Debug.DebugEvent.Break) {
-      exec_state.prepareStep(Debug.StepAction.StepIn, 3);
+      exec_state.prepareStep(Debug.StepAction.StepIn);
     }
   } catch (e) {
+    exception = e;
   }
 }
 
 Debug.setListener(listener);
 
 function f(x) {
   if (x > 0) %_Call(f, null, x-1);
 }
 
 debugger;
 f(2);
 
 Debug.setListener(null);
+assertNull(exception);