nsTextFrame now has only a next-in-flow pointer and no prev-in-flow pointer. Subclass nsContinuingTextFrame is used for continuing text frames; it has a prev-in-flow pointer. This saves 4 bytes git-svn-id: svn://10.0.0.236/trunk@51664 18797224-902f-48f8-a5cc-f745e15eee43