[debugger] remove step count parameter from prepare step.

test/mjsunit/es6/debug-promises/stepin-constructor.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
 
 var Debug = debug.Debug;
 var exception = null;
 var breaks = [];
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     breaks.push(exec_state.frame(0).sourceLineText().trimLeft());
-    exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+    exec_state.prepareStep(Debug.StepAction.StepIn);
   } catch (e) {
     exception = e;
   }
 }
 
 Debug.setListener(listener);
 
 function resolver(resolve, reject) {
   1;
   2;
@@ skipping 12 matching lines @@
   "1;",
   "2;",
   "3;",
   "resolve();",
   "}",
   "Debug.setListener(null);"
 ];
 
 assertEquals(expected_breaks, breaks);
 assertNull(exception);