[debugger] remove step count parameter from prepare step.

test/mjsunit/debug-step-turbofan.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 --turbo-filter=g --allow-natives-syntax
 
 // Test that Debug::PrepareForBreakPoints can deal with turbofan code (g)
 // on the stack.  Without deoptimization support, we will not be able to
 // replace optimized code for g by unoptimized code with debug break slots.
 // This would cause stepping to fail (V8 issue 3660).
@@ skipping 15 matching lines @@
   debugger;               // Break 0
 }                         // Break 1
 
 Debug = debug.Debug;
 var exception = null;
 var break_count = 0;
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
-    exec_state.prepareStep(Debug.StepAction.StepNext, 1);
+    exec_state.prepareStep(Debug.StepAction.StepNext);
     print(exec_state.frame(0).sourceLineText());
     var match = exec_state.frame(0).sourceLineText().match(/Break (\d)/);
     assertNotNull(match);
     assertEquals(break_count++, parseInt(match[1]));
   } catch (e) {
     print(e + e.stack);
     exception = e;
   }
 }
 
 f(0);
 f(0);
 %OptimizeFunctionOnNextCall(g);
 
 Debug.setListener(listener);
 
 f(1);
 
 Debug.setListener(null);  // Break 9
 assertNull(exception);
 assertEquals(10, break_count);