[debugger] remove step count parameter from prepare step.

test/mjsunit/es6/debug-stepin-string-template.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
 
 Debug = debug.Debug
 
 var exception = null;
 var log = [];
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     print(event_data.sourceLineText());
     var entry = "";
     for (var i = 0; i < exec_state.frameCount(); i++) {
       entry += exec_state.frame(i).sourceLineText().substr(-1);
       entry += exec_state.frame(i).sourceColumn();
     }
     log.push(entry);
-    exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+    exec_state.prepareStep(Debug.StepAction.StepIn);
   } catch (e) {
     exception = e;
   }
 };
 
 function u(x) {
   return x.toUpperCase();                         // d
 }                                                 // e
 
 var n = 3;
@@ skipping 18 matching lines @@
   "b0",
   "d2b13",
   "e0b13",
   "b25",
   "d2b25",
   "e0b25",
   "f4b44",
   "g2b44",
   "c0"
 ], log);