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

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https://hg.mozilla.org/integration/gaia-central/rev/75e1c4ee15f4
Author: Hubert Figuière <hub@figuiere.net>
Desc: Merge pull request #25191 from hfiguiere/bug1018491-search

Search improvements - Bug 838035 and bug 1079349 - r=squib

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https://hg.mozilla.org/integration/gaia-central/rev/c3c3a5b45b99
Author: Hubert Figuière <hub@figuiere.net>
Desc: Bug 838035 - Part 2: tests for the search context

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https://hg.mozilla.org/integration/gaia-central/rev/4655b75dde7c
Author: Hubert Figuière <hub@figuiere.net>
Desc: Bug 838035 - Part 1: Search context

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https://hg.mozilla.org/integration/gaia-central/rev/8047f5159860
Author: Hubert Figuière <hub@figuiere.net>
Desc: Bug 1079349 - Some integration tests for search.
Bumping manifests a=b2g-bump