Replace NULL with nullptr in all C++ files.

webrtc/modules/desktop_capture/x11/x_server_pixel_buffer.cc

Index: webrtc/modules/desktop_capture/x11/x_server_pixel_buffer.cc
diff --git a/webrtc/modules/desktop_capture/x11/x_server_pixel_buffer.cc b/webrtc/modules/desktop_capture/x11/x_server_pixel_buffer.cc
index 2ecac6b8b0321c87da9645770942fd9f827507c9..4bef0a7c75f3e2f921acbb76a0afe2bcd0586f62 100644
--- a/webrtc/modules/desktop_capture/x11/x_server_pixel_buffer.cc
+++ b/webrtc/modules/desktop_capture/x11/x_server_pixel_buffer.cc
@@ -68,7 +68,7 @@ XServerPixelBuffer::~XServerPixelBuffer() {
 void XServerPixelBuffer::Release() {
   if (x_image_) {
     XDestroyImage(x_image_);
-    x_image_ = NULL;
+    x_image_ = nullptr;
   }
   if (shm_pixmap_) {
     XFreePixmap(display_, shm_pixmap_);
@@ -76,7 +76,7 @@ void XServerPixelBuffer::Release() {
   }
   if (shm_gc_) {
     XFreeGC(display_, shm_gc_);
-    shm_gc_ = NULL;
+    shm_gc_ = nullptr;
   }
   if (shm_segment_info_) {
     if (shm_segment_info_->shmaddr != reinterpret_cast<char*>(-1))
@@ -84,7 +84,7 @@ void XServerPixelBuffer::Release() {
     if (shm_segment_info_->shmid != -1)
       shmctl(shm_segment_info_->shmid, IPC_RMID, 0);
     delete shm_segment_info_;
-    shm_segment_info_ = NULL;
+    shm_segment_info_ = nullptr;
   }
   window_ = 0;
 }