[debugger] remove step count parameter from prepare step.

test/mjsunit/harmony/debug-stepin-default-parameters.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 --harmony-default-parameters
 
 Debug = debug.Debug
 
 var exception = null;
 var log = [];
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     print(event_data.sourceLineText());
     var entry = "";
     for (var i = 0; i < exec_state.frameCount(); i++) {
       entry += exec_state.frame(i).sourceLineText().substr(-1);
       entry += exec_state.frame(i).sourceColumn();
     }
     log.push(entry);
-    exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+    exec_state.prepareStep(Debug.StepAction.StepIn);
   } catch (e) {
     exception = e;
   }
 };
 
 function default_arg(x) {
   return "default";                 // d
 }                                   // e
 
 function f(arg0 = default_arg()) {  // f
   return arg0;                      // g
 }                                   // h
 
 
 Debug.setListener(listener);
 debugger;                           // a
 var result = f();                   // b
 Debug.setListener(null);            // c
 
 assertNull(exception);
 assertEquals("default", result);
 
 assertEquals(["a0","b0","f18b13","d2f18b13","e0f18b13","g2b13","h0b13","c0"],
              log);