Replace NULL with nullptr in all C++ files.

webrtc/pc/sctputils.cc

Index: webrtc/pc/sctputils.cc
diff --git a/webrtc/pc/sctputils.cc b/webrtc/pc/sctputils.cc
index 1c2f76f496d5ea87d573f0e526ba6855493eb12f..5ab71f66d7b502e5356b47afdea154ff9bc278f8 100644
--- a/webrtc/pc/sctputils.cc
+++ b/webrtc/pc/sctputils.cc
@@ -164,9 +164,9 @@ bool WriteDataChannelOpenMessage(const std::string& label,
     }
   }
 
-  rtc::ByteBufferWriter buffer(
-      NULL, 20 + label.length() + config.protocol.length(),
-      rtc::ByteBuffer::ORDER_NETWORK);
+  rtc::ByteBufferWriter buffer(nullptr,
+                               20 + label.length() + config.protocol.length(),
+                               rtc::ByteBuffer::ORDER_NETWORK);
   // TODO(tommi): Add error handling and check resulting length.
   buffer.WriteUInt8(DATA_CHANNEL_OPEN_MESSAGE_TYPE);
   buffer.WriteUInt8(channel_type);