Bumping gaia.json for 1 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/d9c7a3149557
Author: Vivien Nicolas <vnicolas@mozilla.com>
Desc:  Bug 1029853 - Hide open browser screens when browser invoked via activity. r=daleharvey
Bumping manifests a=b2g-bump