Replace NULL with nullptr in all C++ files.

webrtc/modules/rtp_rtcp/source/rtp_format.cc

Index: webrtc/modules/rtp_rtcp/source/rtp_format.cc
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_format.cc b/webrtc/modules/rtp_rtcp/source/rtp_format.cc
index 753fc2ec41795684f1b7d709416ed0ad1e94a931..a23302e4f2341cde125913070d490f6e41cc132c 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_format.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtp_format.cc
@@ -38,7 +38,7 @@ RtpPacketizer* RtpPacketizer::Create(RtpVideoCodecTypes type,
     case kRtpVideoNone:
       RTC_NOTREACHED();
   }
-  return NULL;
+  return nullptr;
 }
 
 RtpDepacketizer* RtpDepacketizer::Create(RtpVideoCodecTypes type) {
@@ -54,6 +54,6 @@ RtpDepacketizer* RtpDepacketizer::Create(RtpVideoCodecTypes type) {
     case kRtpVideoNone:
       assert(false);
   }
-  return NULL;
+  return nullptr;
 }
 }  // namespace webrtc