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

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https://hg.mozilla.org/integration/gaia-central/rev/6848f14fe19d
Author: Fernando Jiménez Moreno <ferjmoreno@gmail.com>
Desc: Merge pull request #12578 from ferjm/bug920851.mozbrowser.trustedui

Bug 920851 - Listen to mozbrowser events on iframes embedded within the ...

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https://hg.mozilla.org/integration/gaia-central/rev/b50b0331c80e
Author: Fernando Jiménez <ferjmoreno@gmail.com>
Desc: Bug 920851 - Listen to mozbrowser events on iframes embedded within the trusted UI instead of on the trusted UI iframe itself. r=vingtetun