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

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https://hg.mozilla.org/integration/gaia-central/rev/ac0ff2fff813
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #12002 from KevinGrandon/bug_908947_usage_dom

Bug 908947 - Use HTML import polyfill for Usage DOM r=salva

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https://hg.mozilla.org/integration/gaia-central/rev/2b6d0a31001d
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 908947 - Use HTML import polyfill for Usage DOM