Bumping gaia.json for 3 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/99de94db4288
Author: Gabriele Svelto <gsvelto@mozilla.com>
Desc: Bug 976503 - Properly handle incoming messages with a 'si-id' field r=julienw

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https://hg.mozilla.org/integration/gaia-central/rev/3e34309e9e47
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #16943 from vingtetun/bug978065

Bug 978065 - [Music] Favor scroll performance over thumbnails quality. r...

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https://hg.mozilla.org/integration/gaia-central/rev/32dac9a7110a
Author: Vivien Nicolas <21@vingtetun.org>
Desc: Bug 978065 - [Music] Favor scroll performance over thumbnails quality. r=dkuo
Bumping manifests a=b2g-bump