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

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https://hg.mozilla.org/integration/gaia-central/rev/b4273bddd6b1
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #20049 from etiennesegonzac/bug-992085-simple

Bug 992085 -  While swiping, enable user to remain in portrait mode when... r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/db1fd010f19d
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 992085 -  While swiping, enable user to remain in portrait mode when  a landscape app is encountered and vice versa.
Bumping manifests a=b2g-bump