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

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https://hg.mozilla.org/integration/gaia-central/rev/4393a8d719e7
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #19301 from etiennesegonzac/bug-1011262

Bug 1011262 - Making sure the layour manager is aware of the new active app before resizing it r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/521599877afe
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 1011262 - Making sure the layour manager is aware of the new active app before resizing it.
Bumping manifests a=b2g-bump