Replace NULL with nullptr in all C++ files.

webrtc/test/linux/video_renderer_linux.cc

Index: webrtc/test/linux/video_renderer_linux.cc
diff --git a/webrtc/test/linux/video_renderer_linux.cc b/webrtc/test/linux/video_renderer_linux.cc
index 6f69dd749804809464414038b5c2e117732b1f6f..c5fb5f8fedf34cfbeb5875c9326681fe4c764e44 100644
--- a/webrtc/test/linux/video_renderer_linux.cc
+++ b/webrtc/test/linux/video_renderer_linux.cc
@@ -18,10 +18,10 @@ VideoRenderer* VideoRenderer::CreatePlatformRenderer(const char* window_title,
                                                      size_t width,
                                                      size_t height) {
   GlxRenderer* glx_renderer = GlxRenderer::Create(window_title, width, height);
-  if (glx_renderer != NULL) {
+  if (glx_renderer != nullptr) {
     return glx_renderer;
   }
-  return NULL;
+  return nullptr;
 }
 }  // test
 }  // webrtc