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

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https://hg.mozilla.org/integration/gaia-central/rev/7ea6c58735e8
Author: steveck-chung <schung@mozilla.com>
Desc: Merge pull request #19598 from pivanov/bug-1008127

Bug 1008127 - [Messages][Refresh] Subject handling in the Composer. r=steveck-chung

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https://hg.mozilla.org/integration/gaia-central/rev/899689cb2193
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 1008127 - [Messages][Refresh] Subject handling in the Composer
Bumping manifests a=b2g-bump