[debugger] remove step count parameter from prepare step.

test/mjsunit/es6/regress/regress-468661.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 break_count = 0;
 
@@ skipping 26 matching lines @@
                  "Expected: // Break " + break_count + ".");
       if (expected_values[break_count] === ReferenceError) {
         assertTrue(v instanceof ReferenceError);
       } else {
         assertSame(expected_values[break_count], v);
 
       }
       ++break_count;
 
       if (break_count !== expected_breaks) {
-        exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+        exec_state.prepareStep(Debug.StepAction.StepIn);
         print("Next step prepared");
       }
     }
   } catch(e) {
     exception = e;
     print(e, e.stack);
   }
 };
 
 Debug.setListener(listener);
 
 var sum = 0;
 (function (){
   'use strict';
 
   debugger; // Break 0.
 
   for (let i=0; // Break 1.
        i < 1;   // Break 2.  // Break 6.
        i++) {   // Break 5.
     let key = i; // Break 3.
     sum += key;   // Break 4.
   }
 }()); // Break 7.
 
 assertNull(exception); // Break 8.
 assertEquals(expected_breaks, break_count);
 
 Debug.setListener(null);