Replace NULL with nullptr in all C++ files.

webrtc/modules/desktop_capture/x11/shared_x_display.cc

Index: webrtc/modules/desktop_capture/x11/shared_x_display.cc
diff --git a/webrtc/modules/desktop_capture/x11/shared_x_display.cc b/webrtc/modules/desktop_capture/x11/shared_x_display.cc
index c5b9bd914c3bc0ddfb415cd1920571406fed39d7..9a3bd2a268a8aaae625cc48b5dbaced685d0290a 100644
--- a/webrtc/modules/desktop_capture/x11/shared_x_display.cc
+++ b/webrtc/modules/desktop_capture/x11/shared_x_display.cc
@@ -33,10 +33,10 @@ SharedXDisplay::~SharedXDisplay() {
 rtc::scoped_refptr<SharedXDisplay> SharedXDisplay::Create(
     const std::string& display_name) {
   Display* display =
-      XOpenDisplay(display_name.empty() ? NULL : display_name.c_str());
+      XOpenDisplay(display_name.empty() ? nullptr : display_name.c_str());
   if (!display) {
     LOG(LS_ERROR) << "Unable to open display";
-    return NULL;
+    return nullptr;
   }
   return new SharedXDisplay(display);
 }