Replace NULL with nullptr in all C++ files.

webrtc/modules/desktop_capture/screen_capturer_x11.cc

Index: webrtc/modules/desktop_capture/screen_capturer_x11.cc
diff --git a/webrtc/modules/desktop_capture/screen_capturer_x11.cc b/webrtc/modules/desktop_capture/screen_capturer_x11.cc
index b4f3bf575b84e64acb4f3d231c0d024bff50c365..789d7435a23e6fdaa7804abec20fe10c9f6c83c4 100644
--- a/webrtc/modules/desktop_capture/screen_capturer_x11.cc
+++ b/webrtc/modules/desktop_capture/screen_capturer_x11.cc
@@ -143,8 +143,8 @@ bool ScreenCapturerLinux::Init(const DesktopCaptureOptions& options) {
     return false;
   }
 
-  gc_ = XCreateGC(display(), root_window_, 0, NULL);
-  if (gc_ == NULL) {
+  gc_ = XCreateGC(display(), root_window_, 0, nullptr);
+  if (gc_ == nullptr) {
     LOG(LS_ERROR) << "Unable to get graphics context";
     DeinitXlib();
     return false;