replace NULL->nullptr in src/remoting.

remoting/host/ipc_video_frame_capturer.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 "remoting/host/ipc_video_frame_capturer.h"
 
 #include "remoting/host/desktop_session_proxy.h"
 #include "third_party/webrtc/modules/desktop_capture/desktop_frame.h"
 
 namespace remoting {
 
 IpcVideoFrameCapturer::IpcVideoFrameCapturer(
     scoped_refptr<DesktopSessionProxy> desktop_session_proxy)
-    : callback_(NULL),
+    : callback_(nullptr),
       desktop_session_proxy_(desktop_session_proxy),
       capture_pending_(false),
       weak_factory_(this) {
 }
 
 IpcVideoFrameCapturer::~IpcVideoFrameCapturer() {
 }
 
 void IpcVideoFrameCapturer::Start(Callback* callback) {
   DCHECK(!callback_);
   DCHECK(callback);
   callback_ = callback;
   desktop_session_proxy_->SetVideoCapturer(weak_factory_.GetWeakPtr());
 }
 
 void IpcVideoFrameCapturer::Capture(const webrtc::DesktopRegion& region) {
   DCHECK(!capture_pending_);
   capture_pending_ = true;
   desktop_session_proxy_->CaptureFrame();
 }
 
 void IpcVideoFrameCapturer::OnCaptureCompleted(
     scoped_ptr<webrtc::DesktopFrame> frame) {
   DCHECK(capture_pending_);
   capture_pending_ = false;
   callback_->OnCaptureCompleted(frame.release());
 }
 
 }  // namespace remoting