Make WikiPage::$mPreparedEdit public

Partial revert of I2c34baaf8b64b204444efa1bfa6ea356e0124c54

Bug: 60054

Change-Id: Id4e90c612941ff2bdb50d7a68f4e18b004c9f744
(cherry picked from commit 09f84556bd33d2eeca29cf0d01b641030f50fb8a)