[debugger] remove step count parameter from prepare step.

test/mjsunit/regress/debug-prepare-step-in.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 12 matching lines @@
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 // Flags: --expose-debug-as debug --allow-natives-syntax --expose-gc
 // Get the Debug object exposed from the debug context global object.
 Debug = debug.Debug
 
 function breakListener(event, exec_state, event_data, data) {
-  exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+  exec_state.prepareStep(Debug.StepAction.StepIn);
 }
 
 Debug.setListener(breakListener);
 
 var o = {x:function() { return 10; }};
 
 function f(o) {
   var m = "x";
   o[m]();
 }
 
 Debug.setBreakPoint(f, 2, 0);
 
 f(o);
 
 %NotifyContextDisposed();
 function g() {
   gc();
 }
 
 g();
 
 Debug.setListener(null);