TextContent: Normalize newlines in preSaveTransform()

This does the same normalization of newlines that
Parser::preSaveTransform() does. This should be appropriate for any text
content type, especially considering that EditPage uses
WebRequest::getText() which does a less-strict version of this same
transformation.

This also cleans up the code for doing that newline replacement
to be a bit less verbose.

Bug: T142805
Change-Id: I462afcda502f031a8b0360d982ce2398a0383a96