[debugger] remove step count parameter from prepare step.

test/mjsunit/es6/debug-stepin-generators.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
 
 Debug = debug.Debug
 var exception = null;
 var yields = 0;
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     var source = exec_state.frame(0).sourceLineText();
     print(source);
     if (/stop stepping/.test(source)) return;
     if (/yield/.test(source)) yields++;
-    exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+    exec_state.prepareStep(Debug.StepAction.StepIn);
   } catch (e) {
     print(e, e.stack);
     exception = e;
   }
 };
 
 Debug.setListener(listener);
 
 function* g() {
   for (var i = 0; i < 3; ++i) {
     yield i;
   }
 }
 
 var i = g();
 debugger;
 for (var num of g()) {}
 i.next();
 
 print(); // stop stepping
 
 // Not stepped into.
 i.next();
 i.next();
 
 assertNull(exception);
 assertEquals(4, yields);