Replace NULL with nullptr in all C++ files.

webrtc/modules/desktop_capture/mouse_cursor_monitor_null.cc

Index: webrtc/modules/desktop_capture/mouse_cursor_monitor_null.cc
diff --git a/webrtc/modules/desktop_capture/mouse_cursor_monitor_null.cc b/webrtc/modules/desktop_capture/mouse_cursor_monitor_null.cc
index 3a632cc0d9cde11cd419b90c8ec8df3396ecd76f..ca425cbc93cde4a31fa92dcaa6a2c84a90000fc5 100644
--- a/webrtc/modules/desktop_capture/mouse_cursor_monitor_null.cc
+++ b/webrtc/modules/desktop_capture/mouse_cursor_monitor_null.cc
@@ -17,13 +17,13 @@ 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