Bumping gaia.json for 4 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/84a648b9cdab
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #19239 from gasolin/issue-973452

Bug 973452 - [settings] refactor Application storage panel with AMD patt..., r=arthur

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https://hg.mozilla.org/integration/gaia-central/rev/b09b39cc2349
Author: gasolin <gasolin@gmail.com>
Desc: Bug 973452 - [settings] refactor Application storage panel with AMD pattern
- put visibilityChange in onBeforeShow/Hide
- clean DeviceStorageHelper
- use observable to render sizes in panel
- turn app_storage to singleton object and moved to modules/
- panel & app_storage test
- use dom request mock for app_storage test

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https://hg.mozilla.org/integration/gaia-central/rev/5d7da913741b
Author: Staś Małolepszy <stas@mozilla.com>
Desc: Merge pull request #18759 from stasm/1002625-music-once

Bug 1002625 - Clean up bootstrap mozL10n API use in Music. r=dkuo

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https://hg.mozilla.org/integration/gaia-central/rev/f0b924508401
Author: Staś Małolepszy <stas@mozilla.com>
Desc: Bug 1002625 - Clean up bootstrap mozL10n API use in Music. r=dkuo

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "remote": "", 
         "branch": ""
     }, 
-    "revision": "37cd9a62b2cb570985d99a5519b136672614b980", 
+    "revision": "84a648b9cdab87db1ac0ad11b21d61bf2fbd965a", 
     "repo_path": "/integration/gaia-central"
 }