Prevent recursion in the debug event listener.

test/mjsunit/regress/regress-crbug-409614.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;
+var error_count = 0;
+
+function f() {
+  return 0;  // Break
+}
+
+function listener(event, exec_state, event_data, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    if (exec_state.frame(0).sourceLineText().indexOf("Break") <0) {
+      error_count++;
+    }
+    exec_state.prepareStep(Debug.StepAction.StepIn, 2);
+    f();  // We should not break in this call of f().
+  } catch (e) {
+    print(e + e.stack);
+    exception = e;
+  }
+}
+
+Debug.setListener(listener);
+
+debugger;  // Break
+f();
+
+Debug.setListener(null);  // Break
+
+assertNull(exception);
+assertEquals(0, error_count);