[debugger] fix stepping over await calls for ignition generators.

test/mjsunit/harmony/async-debug-step-next-constant.js

 // Copyright 2016 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 --allow-natives-syntax --harmony-async-await
-// Flags: --no-ignition-generators
 
 var Debug = debug.Debug;
 var step_count = 0;
 
 function listener(event, execState, eventData, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     var line = execState.frame(0).sourceLineText();
     print(line);
     var [match, expected_count, step] = /\/\/ B(\d) (\w+)$/.exec(line);
@@ skipping 14 matching lines @@
        await         // B3 StepNext
              5;      // B2 StepNext
   return a;          // B4 StepNext
 }                    // B5 Continue
 
 f();
 
 %RunMicrotasks();
 
 assertEquals(6, step_count);