Bumping gaia.json for 3 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/b68d0a3caae2
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #20778 from KevinGrandon/bug_1027990_search_window_activities

Bug 1027990 - [System] ActivityWindow does not receive proper rearWindow

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https://hg.mozilla.org/integration/gaia-central/rev/3bf378f39cf9
Author: vingtetun <21@vingtetun.org>
Desc: Bug 1027990 - Part 2, SearchWindow activity style fixes r=kgrandon

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https://hg.mozilla.org/integration/gaia-central/rev/8696249dcc7c
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1027990 - [System] ActivityWindow does not receive proper rearWindow r=alive
Bumping manifests a=b2g-bump