Apply exportTransform in backupTextPass.inc

This is intended to help us fix 72348: Wikidata uses exportTransform to 
convert old revision content to the new canonical JSON model. This patch
will make this work with the multi-pass dumping process employed to 
generate dumps for dumps.wikimedia.org.

Bug: 72361
Change-Id: Ie9046d1968efc40a02a0812a536f5ef7176af7d7