Replace NULL with nullptr in all C++ files.

webrtc/modules/desktop_capture/mouse_cursor_monitor_null.cc

 /*
  *  Copyright (c) 2013 The WebRTC 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 in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/modules/desktop_capture/mouse_cursor_monitor.h"
 
 #include <stddef.h>
 
 namespace webrtc {
 
 MouseCursorMonitor* MouseCursorMonitor::CreateForWindow(
     const DesktopCaptureOptions& options,
     WindowId window) {
-  return NULL;
+  return nullptr;
 }
 
 MouseCursorMonitor* MouseCursorMonitor::CreateForScreen(
     const DesktopCaptureOptions& options,
     ScreenId screen) {
-  return NULL;
+  return nullptr;
 }
 
 }  // namespace webrtc