Replace NULL with nullptr in all C++ files.

webrtc/modules/video_coding/frame_buffer.cc

Index: webrtc/modules/video_coding/frame_buffer.cc
diff --git a/webrtc/modules/video_coding/frame_buffer.cc b/webrtc/modules/video_coding/frame_buffer.cc
index e3b46de80d65c8159e43d317a73daf16bff8399f..c185724401958e26e3ae51bca54d68aa1c5b442a 100644
--- a/webrtc/modules/video_coding/frame_buffer.cc
+++ b/webrtc/modules/video_coding/frame_buffer.cc
@@ -89,8 +89,8 @@ VCMFrameBufferEnum VCMFrameBuffer::InsertPacket(
     int64_t timeInMs,
     VCMDecodeErrorMode decode_error_mode,
     const FrameData& frame_data) {
-  assert(!(NULL == packet.dataPtr && packet.sizeBytes > 0));
-  if (packet.dataPtr != NULL) {
+  assert(!(nullptr == packet.dataPtr && packet.sizeBytes > 0));
+  if (packet.dataPtr != nullptr) {
     _payloadType = packet.payloadType;
   }