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

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https://hg.mozilla.org/integration/gaia-central/rev/84a4fd458363
Author: Mason Chang <mason@masonchang.com>
Desc: Merge pull request #19487 from hfiguiere/bug1006129

Bug 1006129 - Overfill scroll ratio test for settings. r=hub,eperelman

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https://hg.mozilla.org/integration/gaia-central/rev/9de50e37b8eb
Author: Hubert Figuière <hub@figuiere.net>
Desc: Bug 1006129 - Overfill scroll ratio test for settings.

Based on work from :mchang.
Bumping manifests a=b2g-bump