[debugger] remove step count parameter from prepare step.

test/mjsunit/debug-stepin-accessor-ic.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
 
 function get() {
   return 3;                       // Break
 }                                 // Break
 
@@ skipping 13 matching lines @@
 }                                 // Break
 
 var break_count = 0;
 var exception = null;
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     var source_line = exec_state.frame(0).sourceLineText();
     assertTrue(source_line.indexOf("// Break") > 0);
-    exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+    exec_state.prepareStep(Debug.StepAction.StepIn);
     break_count++;
   } catch (e) {
     exception = e;
   }
 }
 
 var Debug = debug.Debug;
 Debug.setListener(listener);
 
 debugger;                         // Break
 f();                              // Break
 
 Debug.setListener(null);          // Break
 assertEquals(86, break_count);
 assertNull(exception);