Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/789c9a327287
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #26381 from cctuan/1089951

Bug 1089951 - prevent orientation of homescreen is changed by background app

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https://hg.mozilla.org/integration/gaia-central/rev/791750873221
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 1089951 - [Midori 2.0][Homescreen]The main interface menus overlap when MS quits games
Bumping manifests a=b2g-bump