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

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https://hg.mozilla.org/integration/gaia-central/rev/a12711e1f499
Author: Ghislain 'Aus' Lacroix <aus@noiseport.org>
Desc: Merge pull request #20162 from nullaus/bug1009780

bug 1009780 - Improve open mechanism for downloads.

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https://hg.mozilla.org/integration/gaia-central/rev/16896abd7912
Author: Ghislain 'Aus' Lacroix <aus@mozilla.com>
Desc: bug 1009780 - Improve open mechanism for downloads.

r=crdlc, yury
Bumping manifests a=b2g-bump