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

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https://hg.mozilla.org/integration/gaia-central/rev/d2cd15f3a4be
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #13783 from pivanov/bug-939147

Bug 939147 - [Action Icons - Browser] Remove shadows from action icons

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https://hg.mozilla.org/integration/gaia-central/rev/8192ab2b1c43
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 939147 - Remove shadows from browser action icons