Adjust chat log opening functions to use LLLogChat functions to determine log file

This eliminates the possibility of the history button not opening the log file, which has been reported in the past.
This also corrects cases where log files named with date would not be opened.

indra/newview/llfloaterchat.cpp

@@ -653,9 +653,7 @@ void LLFloaterChat::onClickToggleActiveSpeakers(void* userdata)
 // static
 void LLFloaterChat::onClickChatHistoryOpen(void* userdata)
 {
-	char command[256];
-
-	sprintf(command, "\"%s%s%s\"", gDirUtilp->getPerAccountChatLogsDir().c_str(), gDirUtilp->getDirDelimiter().c_str(), "chat.txt");
+	std::string command("\"" + LLLogChat::makeLogFileName("chat") + "\"");
 	gViewerWindow->getWindow()->ShellEx(command);
 
 	llinfos << command << llendl;