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

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https://hg.mozilla.org/integration/gaia-central/rev/cb5ffe462369
Author: John Hu <johu@mozilla.com>
Desc: Merge pull request #15440 from huchengtw-moz/video/Bug_960845-compress-image

Bug 960845 - compress the png image of bug 869289's patch, r=gduan

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https://hg.mozilla.org/integration/gaia-central/rev/21f3abf4e630
Author: John Hu <johu@mozilla.com>
Desc: Bug 960845 - compress the png image of bug 869289's patch

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "4026943dda937f2840f334926355ac2f808d7eb9", 
+    "revision": "cb5ffe462369690ed0bc604065ac37c0cff1f140", 
     "repo_path": "/integration/gaia-central"
 }