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
Bumping manifests a=b2g-bump