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

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https://hg.mozilla.org/integration/gaia-central/rev/8c87851961c5
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #23289 from KevinGrandon/bug_1052503_reland

Bug 1052503 - [Rocketbar] Update loading progress bar

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https://hg.mozilla.org/integration/gaia-central/rev/fc6e904d79fe
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1052503 - [Rocketbar] Update loading progress bar r=benfrancis

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https://hg.mozilla.org/integration/gaia-central/rev/050b7f8fef71
Author: punamdahiya <punamdahiya@yahoo.com>
Desc: Merge pull request #22827 from ranjithgithub/952561_BrightnessSliderIssue

Bug 952651 - [Gallery][Editor] Fix exposure slider button sliding from previous position r=pdahiya

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https://hg.mozilla.org/integration/gaia-central/rev/97ea6e6d3280
Author: ranjith.gutha <ranjith.gutha@lge.com>
Desc: Bug 952651 - [Gallery][Photo Editor] Fixed slider button sliding from previous position

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "remote": "", 
         "branch": ""
     }, 
-    "revision": "52a7b8d9b991335e9d3387a2c9dde3c05abf578e", 
+    "revision": "8c87851961c5f5cdf3bab497621a7fb6c3888728", 
     "repo_path": "/integration/gaia-central"
 }