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

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https://hg.mozilla.org/integration/gaia-central/rev/fcd75dcef6f1
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #22951 from KevinGrandon/bug_1052497_browser_chrome_loading_color

Bug 1052497 - [Browser] Update black load screen to white in browser while loading a page

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https://hg.mozilla.org/integration/gaia-central/rev/76c8a1280795
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1052497 - Implement pixel checking test with corked/uncorked HTTP server. r=benfrancis

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https://hg.mozilla.org/integration/gaia-central/rev/7465a1094a00
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1052497 - [Browser] Update black load screen to white in browser while loading a page. r=benfrancis
Bumping manifests a=b2g-bump