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

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https://hg.mozilla.org/integration/gaia-central/rev/6525edc3e12c
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #12569 from cctuan/922462

Bug 922462 - [Flatfish] use 1.5x icon from manifest for tablet

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https://hg.mozilla.org/integration/gaia-central/rev/377dfd3c903f
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 922462 - Define various size of icons in manifest instead of use buildscript