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

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https://hg.mozilla.org/integration/gaia-central/rev/20121e6d882b
Author: John Hu <johu@mozilla.com>
Desc: Merge pull request #22917 from huchengtw-moz/bug-1045523-3

Bug 1045523 - customize default window size of responsive web design, r=yurenju

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https://hg.mozilla.org/integration/gaia-central/rev/c77d5efdf9c5
Author: John Hu <im@john.hu>
Desc: Bug 1045523 - customize default window size of responsive web design
Bumping manifests a=b2g-bump