[debugger] remove step count parameter from prepare step.

test/mjsunit/debug-step-end-of-script.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;", "", "debugger;"];
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     assertEquals(expected.shift(), exec_state.frame(0).sourceLineText());
-    exec_state.prepareStep(Debug.StepAction.StepNext, 1);
+    exec_state.prepareStep(Debug.StepAction.StepNext);
   } catch (e) {
     %AbortJS(e + "\n" + e.stack);
   }
 }
 
 Debug.setListener(listener);
 debugger;