Bumping gaia.json for 2 gaia-central revision(s)

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https://hg.mozilla.org/integration/gaia-central/rev/456cbcb07cdb
Author: John Hu <johu@mozilla.com>
Desc: Merge pull request #10780 from huchengtw-moz/hd/Bug_881158_for_master

Bug 881158 - hdpi/xhdpi resolution assets and layout for Video app, r=@rexboy, r=@dale

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https://hg.mozilla.org/integration/gaia-central/rev/31d78ed3e9b1
Author: John Hu <johu@mozilla.com>
Desc: Bug 881158 - hdpi/xhdpi resolution assets and layout for Video app