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

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https://hg.mozilla.org/integration/gaia-central/rev/857544fb070a
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #18989 from etiennesegonzac/bug-1000299-tarako-call-ended-missed

Bug 1000299 - Requesting a high priority wake lock when inserting a missed call in the call log r=gsvelto

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https://hg.mozilla.org/integration/gaia-central/rev/6e86dc8f6bd1
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 1000299 - Requesting a high priority wake lock when inserting a missed call in the call log.

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "remote": "", 
         "branch": ""
     }, 
-    "revision": "0afecc884b5b7f95641a70f85b7452c7c2adc2af", 
+    "revision": "857544fb070a9c0306de2f1bffd237bbbeec7a8b", 
     "repo_path": "/integration/gaia-central"
 }