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

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "remote": "", 
         "branch": ""
     }, 
-    "revision": "aed1eb065c435efafc445df60f2dfe47ebcef1e4", 
+    "revision": "7685bf4c496833ad4544a95b59717167fb832f40", 
     "repo_path": "/integration/gaia-central"
 }