[debugger] remove step count parameter from prepare step.

test/mjsunit/debug-stepout-to-builtin.js

 // Copyright 2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 24 matching lines @@
 var expected_source_line_text = null;
 var expected_function_name = null;
 
 // Simple debug event handler which first time will cause 'step out' action
 // and than check that execution is paused inside function
 // expected_function_name.
 function listener(event, exec_state, event_data, data) {
   try {
     if (event == Debug.DebugEvent.Break) {
       if (state == 1) {
-        exec_state.prepareStep(Debug.StepAction.StepOut, 2);
-        state = 2;
-      } else if (state == 2) {
+        exec_state.prepareStep(Debug.StepAction.StepOut);
+        state++;
+      } else {
         assertEquals(expected_function_name, event_data.func().name());
         assertEquals(expected_source_line_text,
                      event_data.sourceLineText());
         state = 3;
       }
     }
   } catch(e) {
     exception = e;
   }
 };
@@ skipping 17 matching lines @@
   JSON.stringify(obj, replacer);
 }  // expected line
 
 state = 1;
 testStepOutToBuiltIn();
 assertNull(exception);
 assertEquals(3, state);
 
 // Get rid of the debug event listener.
 Debug.setListener(null);