[debugger] remove step count parameter from prepare step.

test/mjsunit/debug-break-native.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
 
 Debug = debug.Debug
 var exception = null;
 
 function breakListener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
-    exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+    exec_state.prepareStep(Debug.StepAction.StepIn);
     // Assert that the break happens at an intended location.
     assertTrue(exec_state.frame(0).sourceLineText().indexOf("// break") > 0);
   } catch (e) {
     exception = e;
   }
 }
 
 Debug.setListener(breakListener);
 
 debugger;                          // break
 
 function f(x) {
   return x;                        // break
 }                                  // break
 
 Debug.setBreakPoint(f, 0, 0);      // break
 Debug.scripts();                   // break
 debug.MakeMirror(f);               // break
 
 new Error("123").stack;            // break
 Math.sin(0);                       // break
 
 f("this should break");            // break
 
 Debug.setListener(null);           // break
 
 f("this should not break");
 
 assertNull(exception);