Replace NULL with nullptr in all C++ files.

webrtc/modules/desktop_capture/mouse_cursor_monitor_x11.cc

Index: webrtc/modules/desktop_capture/mouse_cursor_monitor_x11.cc
diff --git a/webrtc/modules/desktop_capture/mouse_cursor_monitor_x11.cc b/webrtc/modules/desktop_capture/mouse_cursor_monitor_x11.cc
index 64ae44d9d34f25408502432a2aa86d9b5cbf0b4b..8fa9a8963d268c91c38b9fa372caf2f0439e7fc8 100644
--- a/webrtc/modules/desktop_capture/mouse_cursor_monitor_x11.cc
+++ b/webrtc/modules/desktop_capture/mouse_cursor_monitor_x11.cc
@@ -92,7 +92,7 @@ MouseCursorMonitorX11::MouseCursorMonitorX11(
     const DesktopCaptureOptions& options,
     Window window)
     : x_display_(options.x_display()),
-      callback_(NULL),
+      callback_(nullptr),
       mode_(SHAPE_AND_POSITION),
       window_(window),
       have_xfixes_(false),
@@ -237,10 +237,10 @@ void MouseCursorMonitorX11::CaptureCursor() {
 MouseCursorMonitor* MouseCursorMonitor::CreateForWindow(
     const DesktopCaptureOptions& options, WindowId window) {
   if (!options.x_display())
-    return NULL;
+    return nullptr;
   window = GetTopLevelWindow(options.x_display()->display(), window);
   if (window == None)
-    return NULL;
+    return nullptr;
   return new MouseCursorMonitorX11(options, window);
 }
 
@@ -248,7 +248,7 @@ MouseCursorMonitor* MouseCursorMonitor::CreateForScreen(
     const DesktopCaptureOptions& options,
     ScreenId screen) {
   if (!options.x_display())
-    return NULL;
+    return nullptr;
   return new MouseCursorMonitorX11(
       options, DefaultRootWindow(options.x_display()->display()));
 }