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

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https://hg.mozilla.org/integration/gaia-central/rev/12a84f6ff7be
Author: John Hu <johu@mozilla.com>
Desc: Merge pull request #17837 from shamenchens/Bug967089-Remove-Visibility-Monitor-ImgSrc2

Bug 967089 - [gallery] remove visibility monitor code if it is no longer..., r=punam

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https://hg.mozilla.org/integration/gaia-central/rev/f47c57ba856b
Author: Sherman Chen <shamen.chens@gmail.com>
Desc: Bug 967089 - [gallery] remove visibility monitor code if it is no longer needed
Bumping manifests a=b2g-bump