Replace NULL with nullptr in all C++ files.

webrtc/modules/desktop_capture/x11/x_error_trap.cc

Index: webrtc/modules/desktop_capture/x11/x_error_trap.cc
diff --git a/webrtc/modules/desktop_capture/x11/x_error_trap.cc b/webrtc/modules/desktop_capture/x11/x_error_trap.cc
index 458fbe61a092300cc2abe5286360c07dd5508b44..c94a7876b91e453d2b5bfb253a3601a3b93e7107 100644
--- a/webrtc/modules/desktop_capture/x11/x_error_trap.cc
+++ b/webrtc/modules/desktop_capture/x11/x_error_trap.cc
@@ -37,8 +37,7 @@ int XServerErrorHandler(Display* display, XErrorEvent* error_event) {
 }  // namespace
 
 XErrorTrap::XErrorTrap(Display* display)
-    : original_error_handler_(NULL),
-      enabled_(true) {
+    : original_error_handler_(nullptr), enabled_(true) {
 #if defined(TOOLKIT_GTK)
   gdk_error_trap_push();
 #else  // !defined(TOOLKIT_GTK)