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

========

https://hg.mozilla.org/integration/gaia-central/rev/8ac363347c96
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #19843 from eeejay/bug-1018492

Bug 1018492 - Use accessibility roles in gaia-tabs and in example html.

========

https://hg.mozilla.org/integration/gaia-central/rev/b272662f1d93
Author: Eitan Isaacson <eitan@monotonous.org>
Desc: Bug 1018492 - Use accessibility roles in gaia-tabs and in example html.
Bumping manifests a=b2g-bump