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

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https://hg.mozilla.org/integration/gaia-central/rev/851d03d8d01a
Author: Eitan Isaacson <eitan@monotonous.org>
Desc: Merge pull request #15781 from eeejay/bug-952312

Bug 952312 - Screen reader settings. r=kaze

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https://hg.mozilla.org/integration/gaia-central/rev/c52d9667d5f2
Author: Eitan Isaacson <eitan@monotonous.org>
Desc: Bug 952312 - Screen reader settings
Bumping manifests a=b2g-bump