[debugger] remove step count parameter from prepare step.

test/mjsunit/es6/debug-stepnext-for.js

 // Copyright 2014 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 --harmony
 
 Debug = debug.Debug;
 var break_count = 0
 var exception = null;
 var log = []
@@ skipping 54 matching lines @@
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     var line = exec_state.frame(0).sourceLineText();
     var col = exec_state.frame(0).sourceColumn();
     print(line);
     var match = line.match(/\/\/ Break (\w)$/);
     assertEquals(2, match.length);
     log.push(match[1] + col);
-    exec_state.prepareStep(Debug.StepAction.StepNext, 1);
+    exec_state.prepareStep(Debug.StepAction.StepNext);
     break_count++;
   } catch (e) {
     exception = e;
   }
 }
 
 Debug.setListener(listener);
 f();
 Debug.setListener(null);         // Break z
 
@@ skipping 29 matching lines @@
   // For-let: init, condition, body, next, condition, body, ...
   "m7","m20","M4","m26","m20","M4","m26","m20","M4","m26","m20",
   // Exit.
   "y0","z0",
 ]
 print("expected:\n"+ JSON.stringify(expected));
 
 assertArrayEquals(expected, log);
 assertEquals(54, s);
 assertNull(exception);