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

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https://hg.mozilla.org/integration/gaia-central/rev/7685bf4c4968
Author: steveck-chung <schung@mozilla.com>
Desc: Merge pull request #17630 from steveck-chung/bug-983631

Bug 983631 - [Messages] After deleting a message thread the notification persists forever. r=julienw

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https://hg.mozilla.org/integration/gaia-central/rev/b9986a0e8c20
Author: Steve Chung <schung@mozilla.com>
Desc: Bug 983631 - [Messages] After deleting a message thread the notification persists forever.

* Add closeNotificationsForThread api and make it return as promise
* Apply julien's promise-based test case
Bumping manifests a=b2g-bump