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

========

https://hg.mozilla.org/integration/gaia-central/rev/28dafcde8bdb
Author: Borja Salguero <borjasalguero@users.noreply.github.com>
Desc: Merge pull request #13512 from borjasalguero/bug_scrolling_issues

Bug 936370 [Messaging] Scrolling issues and reduce unnecessary calls for updating UI. r=julienw

========

https://hg.mozilla.org/integration/gaia-central/rev/9a511758af83
Author: borjasalguero <fbsc@tid.es>
Desc: Bug 936379 [Messaging] Scrolling issues and reduce unnecessary calls for updating UI. r=rwaldron