Bumping gaia.json for 4 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/a3d8ac8f5520
Author: Ryan VanderMeulen <rvandermeulen@mozilla.com>
Desc: Merge pull request #20435 from eviljeff/patch-1

expose dom.mozApps.use_reviewer_certs preference in settings

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https://hg.mozilla.org/integration/gaia-central/rev/15e6a1977854
Author: Andrew Williamson <awilliamson@mozilla.com>
Desc: Add dom.mozApps.use_reviewer_certs preference toggle

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https://hg.mozilla.org/integration/gaia-central/rev/40704e53e365
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #20884 from etiennesegonzac/bug-1027991-landing

Bug 1027991 - Use internal keyword to create child sheet window r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/12ea307f3fa2
Author: Alive Kuo <alegnadise@gmail.com>
Desc: Bug 1027991 - Use internal keyword to create child sheet window
Bumping manifests a=b2g-bump