[debugger] remove step count parameter from prepare step.

test/mjsunit/regress/regress-crbug-467180.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
 
 function f() {
   for (var i = 10; i < 14; i++) {  // 1
     i;                             // 2
   }
 }                                  // 3
 
 var state = "conditional";
 var log = [];
 var exception = null;
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     var label = +exec_state.frame(0).sourceLineText().substr(-1);
     log.push(label);
     if (label == 2) log.push(exec_state.frame(0).evaluate("i").value());
-    exec_state.prepareStep(Debug.StepAction.StepNext, 1);
+    exec_state.prepareStep(Debug.StepAction.StepNext);
   } catch (e) {
     exception = e;
     print("Caught something. " + e + " " + e.stack);
   };
 };
 
 
 var Debug = debug.Debug;
 Debug.setListener(listener);
 
 Debug.setBreakPoint(f, 2, 0, "i == 12");
 
 f();
 
 Debug.setListener(null);  // 4
 
 assertEquals([2,12,1,1,2,13,1,1,3,4], log);
 assertNull(exception);