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

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https://hg.mozilla.org/integration/gaia-central/rev/b8dd552c61e7
Author: Ben Francis <ben@tola.me.uk>
Desc: Merge pull request #18762 from rnowm/1002501

Bug 1002501 - Regression: Incorrect background colour of disabled back/forward buttons r=benfrancis

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https://hg.mozilla.org/integration/gaia-central/rev/920075e6f426
Author: rnowm <arnau@arnaumarch.com>
Desc: Bug 1002501 - Regression: Incorrect background colour of disabled back/forward buttons
Bumping manifests a=b2g-bump