MAINT-5020 FIXED Determine the need to use bracket via chat source.

indra/newview/llchathistory.cpp

@@ -934,7 +934,7 @@ void LLChatHistory::appendMessage(const LLChat& chat, const LLSD &args, const LL
 	// compact mode: show a timestamp and name
 	if (use_plain_text_chat_history)
 	{
-		square_brackets = chat.mFromName == SYSTEM_FROM;
+		square_brackets = chat.mSourceType == CHAT_SOURCE_SYSTEM;
 
 		LLStyle::Params timestamp_style(body_message_params);