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

========

https://hg.mozilla.org/integration/gaia-central/rev/2b9d3584daf7
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Merge pull request #15527 from timdream/window-activity-chain

Bug 957678 - Fix activityCaller/Callee ref in the chain, r=alive

========

https://hg.mozilla.org/integration/gaia-central/rev/d2ca82557499
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Bug 957678 - Fix activityCaller/Callee ref in the chain
Bumping manifests a=b2g-bump