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

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https://hg.mozilla.org/integration/gaia-central/rev/bc4c53c8506d
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #12858 from vingtetun/focus.out.hot.path

 Bug 926875 - Move the iframe.focus call out of the hot path to avoid an...

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https://hg.mozilla.org/integration/gaia-central/rev/54cc6f3ca1cc
Author: Vivien Nicolas <21@vingtetun.org>
Desc:  Bug 926875 - Move the iframe.focus call out of the hot path to avoid an expensive reflow. r=alive