[debugger] remove step count parameter from prepare step.

test/mjsunit/regress/regress-crbug-119800.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() {
   1;
   2;
   3;
 }
 
 var Debug = debug.Debug;
 var exception = null;
 var breaks = [];
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     Debug.debuggerFlags().breakPointsActive.setValue(false);
     breaks.push(exec_state.frame().sourceLineText().trimLeft());
-    exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+    exec_state.prepareStep(Debug.StepAction.StepIn);
   } catch (e) {
     exception = e;
   }
 }
 
 Debug.setListener(listener);
 Debug.setBreakPoint(f, 0, 0);
 
 f();
 
 Debug.setListener(null);
 Debug.debuggerFlags().breakPointsActive.setValue(true);
 
 assertNull(exception);
 assertEquals(breaks, ["1;", "2;", "3;", "}", "Debug.setListener(null);"]);