[chromedriver] Allow commands to be async.

chrome/test/chromedriver/command.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_TEST_CHROMEDRIVER_COMMAND_H_
 #define CHROME_TEST_CHROMEDRIVER_COMMAND_H_
 
 #include <string>
 
 #include "base/callback_forward.h"
 #include "base/memory/scoped_ptr.h"
 
 namespace base {
 class DictionaryValue;
 class Value;
 }
 
 class Status;
 
-typedef base::Callback<Status(
+typedef base::Callback<void(
+    const Status&,
+    scoped_ptr<base::Value>,
+    const std::string&)> CommandCallback;
+
+typedef base::Callback<void(
     const base::DictionaryValue&,
     const std::string&,
-    scoped_ptr<base::Value>*,
-    std::string*)> Command;
+    const CommandCallback&)> Command;
 
 #endif  // CHROME_TEST_CHROMEDRIVER_COMMAND_H_