Cleanup usage of scoped_ptr<> in remoting for C++11

remoting/host/audio_capturer_mac.cc

 // 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.
 
 #include "base/logging.h"
 #include "remoting/host/audio_capturer.h"
 
 namespace remoting {
 
 bool AudioCapturer::IsSupported() {
   return false;
 }
 
 scoped_ptr<AudioCapturer> AudioCapturer::Create() {
   NOTIMPLEMENTED();
-  return scoped_ptr<AudioCapturer>();
+  return nullptr;
 }
 
 }  // namespace remoting