Replace NULL with nullptr in all C++ files.

webrtc/modules/video_coding/generic_encoder.cc

Index: webrtc/modules/video_coding/generic_encoder.cc
diff --git a/webrtc/modules/video_coding/generic_encoder.cc b/webrtc/modules/video_coding/generic_encoder.cc
index 41f2cad6ffec1fa9f545556da96575a2e2b1d3b4..c4679e4376673081848bdd9d6bb21ad64ec28ef3 100644
--- a/webrtc/modules/video_coding/generic_encoder.cc
+++ b/webrtc/modules/video_coding/generic_encoder.cc
@@ -128,9 +128,9 @@ int32_t VCMGenericEncoder::RequestFrame(
   // TODO(nisse): Used only with internal source. Delete as soon as
   // that feature is removed. The only implementation I've been able
   // to find ignores what's in the frame.
-  return encoder_->Encode(VideoFrame(I420Buffer::Create(1, 1),
-                                     kVideoRotation_0, 0),
-                          NULL, &frame_types);
+  return encoder_->Encode(
+      VideoFrame(I420Buffer::Create(1, 1), kVideoRotation_0, 0), nullptr,
+      &frame_types);
   return 0;
 }