Replace NULL with nullptr in all C++ files.

webrtc/media/base/rtpdataengine.cc

Index: webrtc/media/base/rtpdataengine.cc
diff --git a/webrtc/media/base/rtpdataengine.cc b/webrtc/media/base/rtpdataengine.cc
index 96f5a3b95fc70d3edbde74fdb230779648b4170d..43c689eb801574415e10c7504ddb579f411d80bd 100644
--- a/webrtc/media/base/rtpdataengine.cc
+++ b/webrtc/media/base/rtpdataengine.cc
@@ -87,7 +87,7 @@ const DataCodec* FindUnknownCodec(const std::vector<DataCodec>& codecs) {
       return &(*iter);
     }
   }
-  return NULL;
+  return nullptr;
 }
 
 const DataCodec* FindKnownCodec(const std::vector<DataCodec>& codecs) {
@@ -98,7 +98,7 @@ const DataCodec* FindKnownCodec(const std::vector<DataCodec>& codecs) {
       return &(*iter);
     }
   }
-  return NULL;
+  return nullptr;
 }
 
 bool RtpDataMediaChannel::SetRecvCodecs(const std::vector<DataCodec>& codecs) {